[3.x] Add MOUSE_MODE_CONFINED_HIDDEN to MouseMode enum

[aaronfranke]

This PR backports #43450 to Godot 3.x, since it was requested.

I have tested that this works on macOS, but this needs testing for the other platforms.

Minimal test script:

func _ready():
	Input.set_mouse_mode(Input.MOUSE_MODE_CONFINED_HIDDEN)

[akien-mga]

Tested on Linux X11, seems to work as expected.

[akien-mga]

Thanks!