Improve the command-palette

[CamilleLetavernier]

Original issue: eclipsesource/ecore-glsp#97

When I select a class or interface or any other basic node in the diagram and press ctrl + space an input opens.
After some time a dropdown shows up with some entries that look like UIDs.

Output in the GLSP-Server:

SEVERE: Unknown action kind: requestCommandPaletteActions
java.lang.IllegalArgumentException: Unknown action kind: requestCommandPaletteActions
	at com.eclipsesource.glsp.api.json.ActionTypeAdapter.createInstance(ActionTypeAdapter.java:43)
	at com.eclipsesource.glsp.api.json.ActionTypeAdapter.createInstance(ActionTypeAdapter.java:30)
	at com.eclipsesource.glsp.graph.gson.PropertyBasedTypeAdapter.read(PropertyBasedTypeAdapter.java:59)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:129)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:220)
	at com.google.gson.Gson.fromJson(Gson.java:888)
	at org.eclipse.lsp4j.jsonrpc.json.adapters.MessageTypeAdapter.fromJson(MessageTypeAdapter.java:329)
	at org.eclipse.lsp4j.jsonrpc.json.adapters.MessageTypeAdapter.parseParams(MessageTypeAdapter.java:249)
	at org.eclipse.lsp4j.jsonrpc.json.adapters.MessageTypeAdapter.read(MessageTypeAdapter.java:119)
	at org.eclipse.lsp4j.jsonrpc.json.adapters.MessageTypeAdapter.read(MessageTypeAdapter.java:55)
	at com.google.gson.Gson.fromJson(Gson.java:888)
	at org.eclipse.lsp4j.jsonrpc.json.MessageJsonHandler.parseMessage(MessageJsonHandler.java:119)
	at org.eclipse.lsp4j.jsonrpc.json.MessageJsonHandler.parseMessage(MessageJsonHandler.java:114)
	at org.eclipse.lsp4j.jsonrpc.json.StreamMessageProducer.handleMessage(StreamMessageProducer.java:193)
	at org.eclipse.lsp4j.jsonrpc.json.StreamMessageProducer.listen(StreamMessageProducer.java:94)
	at org.eclipse.lsp4j.jsonrpc.json.ConcurrentMessageProcessor.run(ConcurrentMessageProcessor.java:113)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:830)

Ctrl + Space opens the commands palette (similar to Theia's F1, but scoped to the current selection in the diagram). The default palette should contain "Select " entries for each node in the diagram, but it seems broken (I might have broken it in the recent server changes, because it still worked very recently). The problem is that it showed UUIDs instead of Node label, making it unusable in practice.

I think we should keep the command palette, but:

• Restore & Fix the "Select" commands
• Consider other commands (e.g. "Create attribute" for a selected Class, "Create literal" for a selected Enum, ..?)