[WIP] Implement MenuBar control to wrap Popup menus or native menu, use native menu for editor.

[bruvzg]

• Implements MenuBar control, that can use either collection of PopupMenu or OS native global menu.

TODO

• [X] Implement MenuBar control
• [X] Convert main editor menu to MenuBar
• [ ] Convert all sub-editor menus to MenuBar
  • [X] ScriptEditorPlugin
• [ ] Test on all platforms
• [ ] Add documentation

Previews - click to expand

MenuBar with native menu enabled:

https://user-images.githubusercontent.com/7645683/183052878-a1ad5adc-6fe9-4ad0-bb74-1f3e9beb040b.mov

MenuBar with native menu disabled:

https://user-images.githubusercontent.com/7645683/183052905-95699b77-ba3f-4a93-87e9-817407d5bb78.mov

Supersede #38044 Implements https://github.com/godotengine/godot-proposals/issues/736