Move global_menu_* methods to a separate NativeMenu class.

[bruvzg]

• Move global menu related methods to the separate NativeMenu class and change API to use RIDs instead of string identifiers.
• Add methods to get native menu size and get/set custom min. width.
• Add method to popup native menu at arbitrary location (useful for Implement support for application status indicators (tray icons). #80211).

Implements godotengine/godot-proposals#8685

[YuriSizov]

Since we change the API we should mark it as breaking compat, even if we have compatibility methods. But it's a very much welcome change!

[Riteo]

What's the point of this? Can't we just add the variable and check later, if we ever add proper native menu support?

[bruvzg]

Some dummy instance should exist, since it can be accessed in any platform. Also, even if we won't have global menu support we might implement native popup menu at some point (can be useful for the tray indicator menus, and I think the only way tray indicators can be done on X11).

[Mickeon]

The existing methods in DisplayServer should be marked as deprecated, shouldn't they? I don't see this being done in the current PR.

[Mickeon]

Docs are decent. They could be worded better but that's outside the scope of the PR, since the descriptions are all copy-pasted. What matters more is that the DisplayServer's methods are deprecated properly.

[akien-mga]

Looks really good to me!

[akien-mga]

Thanks!