Copy/Cut/Paste context commands are rendered twice in the monaco context menu

[safisa]

Bug Description:

Hi,
See the attached screenshot.

Steps to Reproduce:

1. Run the browser sample in the latest Theia (master), and open the context menu in any Monaco editor.
2. See that Copy/Cut/Paste are duplicated in the context menu.

Additional Information

• Operating System: Mac using Chrome
• Theia Version: latest (master)

[tsmaeder]

The method MenuModelRegistry.findInNode() is bogus at line 319: it will only ever find compound menu nodes. This has the effect that we're both adding the "cut" action from monaco and the theia implementation since the existence check in monaco-menu.ts#52 is false when it should be true.

[safisa]

@tsmaeder there is also an issue in the git/scm context menu too, could be the same issue above:

The open file, discard changes, and stage chnages are duplicated:

[tsmaeder]

For example "Discard Changes" is still duplicated when using "custom" title bar style, works with "native".