Make the editor toolbar buttons configurable

[TurkeyMan]

• VSCode Version: 1.3.1
• OS Version: Arch

Split editor has a dedicated button at the top-right of the editor. That functionality has never been a button I want. What I really do want very frequently though, is a move-editor-right button in that same place.
It would be nice if we could configure the button(/s) in that location.

[bpasero]

In theory with the work @jrieken does we could add an alternate action that does the move instead of split (e.g. when you press the Alt key while hovering over the button).

[jrieken]

Maybe a dupe of #9285. The ideas is that we will have something like menus.json which allows users to go the full mile. Challenge is that we still miss some underlying refactorings to really enable this

[gko]

Any updates on this?

[jrieken]

Closing this as duplicate of #9285