Rearrange items on MenuButton

[Nukiloco]

Godot version:

Godot v3.1

OS/device including version:

Windows 10

Issue description:

For some reason you can't rearrange the order of the items on the MenuButton when using the item list editor so instead you have to remove all the items and add them all back in just to rearrange the item list.

[Calinou]

Closing in favor of godotengine/godot-proposals#368, godotengine/godot-proposals#1975, godotengine/godot-proposals#2355 and godotengine/godot-proposals#2356, as feature proposals are now tracked on the Godot proposals repository.