plots: Dynamic plots not removed when language runtime shuts down

[jmcphers]

To reproduce:

1. Start R and open the Plots pane
2. Create a plot with e.g. plot(runif(1000))
3. Exit R, with either q() or with the Shutdown Language Runtime command

The Plots pane still shows the plot.

The plot shouldn't be visible any more, since its backend has been removed (comm closed). If you try to resize the Plots pane you'll see it try to redraw but get stuck because the backend is disconnected (note heavy blue line in screenshot).

[jmcphers]

Thinking about this a little more ... I'm not sure that aggressively removing the plots from the pane is the correct behavior. We can't resize them any more, but we could still show the last rendered image without much trouble.

[jmcphers]

This is almost certainly related to the error @DavisVaughan noted in another issue, in which we try to disconnect the comm after the kernel has already shut down.

ANSIOutput: Processing Output: "onDidChangeRuntimeState (idle)"
ansiOutput.ts:220
ANSIOutput: Processing Output: "onDidChangeRuntimeState (exiting)"
ansiOutput.ts:220
ANSIOutput: Processing Output: "onDidChangeRuntimeState (exited)"
ansiOutput.ts:220
ANSIOutput: Processing Output: "R: /Library/Frameworks/R.framework/Resources 4.2.2 has exited."
ansiOutput.ts:220
  ERR Cannot read properties of null (reading 'title'): TypeError: Cannot read properties of null (reading 'title')
	at JupyterKernel.sendToSocket (/Users/davis/Desktop/programming/positron/positron/extensions/jupyter-adapter/out/JupyterKernel.js:695:57)
	at JupyterKernel.send (/Users/davis/Desktop/programming/positron/positron/extensions/jupyter-adapter/out/JupyterKernel.js:676:21)
	at JupyterKernel.closeComm (/Users/davis/Desktop/programming/positron/positron/extensions/jupyter-adapter/out/JupyterKernel.js:439:14)
	at LanguageRuntimeAdapter.removeClient (/Users/davis/Desktop/programming/positron/positron/extensions/jupyter-adapter/out/LanguageRuntimeAdapter.js:251:26)
	at ExtHostLanguageRuntime.$removeClient (/Users/davis/Desktop/programming/positron/positron/out/vs/workbench/api/common/positron/extHostLanguageRuntime.js:74:36)
	at RPCProtocol._doInvokeHandler (/Users/davis/Desktop/programming/positron/positron/out/vs/workbench/services/extensions/common/rpcProtocol.js:381:27)
	at RPCProtocol._invokeHandler (/Users/davis/Desktop/programming/positron/positron/out/vs/workbench/services/extensions/common/rpcProtocol.js:366:45)
	at RPCProtocol._receiveRequest (/Users/davis/Desktop/programming/positron/positron/out/vs/workbench/services/extensions/common/rpcProtocol.js:307:32)
	at RPCProtocol._receiveOneMessage (/Users/davis/Desktop/programming/positron/positron/out/vs/workbench/services/extensions/common/rpcProtocol.js:234:26)
	at /Users/davis/Desktop/programming/positron/positron/out/vs/workbench/services/extensions/common/rpcProtocol.js:115:52
	at Listener.invoke (/Users/davis/Desktop/programming/positron/positron/out/vs/base/common/event.js:660:27)
	at PrivateEventDeliveryQueue.deliver (/Users/davis/Desktop/programming/positron/positron/out/vs/base/common/event.js:836:38)
	at Emitter.fire (/Users/davis/Desktop/programming/positron/positron/out/vs/base/common/event.js:800:37)
	at BufferedEmitter.fire (/Users/davis/Desktop/programming/positron/positron/out/vs/base/parts/ipc/common/ipc.net.js:504:35)
	at /Users/davis/Desktop/programming/positron/positron/out/vs/workbench/api/node/extensionHostProcess.js:180:41
	at Listener.invoke (/Users/davis/Desktop/programming/positron/positron/out/vs/base/common/event.js:660:27)
	at PrivateEventDeliveryQueue.deliver (/Users/davis/Desktop/programming/positron/positron/out/vs/base/common/event.js:836:38)
	at Emitter.fire (/Users/davis/Desktop/programming/positron/positron/out/vs/base/common/event.js:800:37)
	at BufferedEmitter.fire (/Users/davis/Desktop/programming/positron/positron/out/vs/base/parts/ipc/common/ipc.net.js:504:35)
	at MessagePortMain.<anonymous> (/Users/davis/Desktop/programming/positron/positron/out/vs/workbench/api/node/extensionHostProcess.js:77:57)
	at MessagePortMain.emit (node:events:513:28)
	at MessagePortMain._internalPort.emit (node:electron/js2c/utility_init:2:367)
	at Object.callbackTrampoline (node:internal/async_hooks:130:17)

[jmcphers]

I'm not sure that aggressively removing the plots from the pane is the correct behavior. We can't resize them any more, but we could still show the last rendered image without much trouble.

I thought about this a little more and decided that it is the correct behavior, at least for now. It would be easy to show the last rendered image, but there's no place for us to actually store that image today. Consequently, the image would vanish if you reloaded the browser.

It is admittedly a little aggressive to remove the image from the pane right away, but I think that users could understand losing plots when they take a destructive action (shutdown, restart) more than they could losing plots on a UI reload, which should never cause you to lose data.

We will eventually need some storage for the Plots pane for thumbnails if nothing else (see #397), but until we have that set up, I think removal is what we want so that the state remains consistent.

[jmcphers]

This now works in my testing.