[editor] support splitting context menu items for editors in the main area

[vince-fugnitto]

Description

I propose we add the following context-menu items to editors present in the main area, to allow users to quickly split editors:

• split up
• split down
• split left
• split right

In the future we can also assign keybindings to these actions. For reference, they are supported in VS Code:

[akosyakov]

It is fine, but what you will show? Does it depend on https://github.com/eclipse-theia/theia/issues/3857? What about other widgets?

[vince-fugnitto]

Yes I do think it depends on #3857, and I imagined it only for editors like in VS Code.

[vince-fugnitto]

It is fine, but what you will show? Does it depend on #3857? What about other widgets?

@akosyakov I also had in mind to support moving editors through a context-menu and keybinding. Instead of having to drag & drop, it'd split and move the editor. Do you think it's a worthwhile feature to support? I believe versions of Jetbrains IDE have the same functionality.

[akosyakov]

It would be fine with me, but you are better ask others. Generally being able to do everything with the keyboard is good. Maybe open another issue and ping other committers?

[vince-fugnitto]

Closing as the commands are present :+1: