Right click menu

[davidanthoff]

When one clicks with the right mouse on a thumbnail we should show a context menu that for now has one option, namely "Delete" that removes that plot.

[TonyLianLong]

The code that works for me is here: https://electronjs.org/docs/api/menu#render-process

Since we need to run require to get electron functions, we have to detect whether we are using electron so that we can continue debugging with a browser. This package is probably a good way to do it: https://github.com/cheton/is-electron.

[TonyLianLong]

I'll work on this issue as well as #14 and julia-vscode/plotgallery#10 first.