Toggle fullscreen mode when pressing Alt + Enter by default

[Calinou]

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Alt + Enter is a widely followed convention to toggle fullscreen. This adds support for it both in the editor and projects, as long as the window is configured to be resizable.

The rationale for providing this feature in all projects is that many developers don't bother about adding a fullscreen toggle, especially for smaller/gamejam games. Yet, this convention is followed by many popular games out there, including AAA games.

The shortcut can be modified or disabled by editing the Input Map in the Project Settings.

This closes https://github.com/godotengine/godot-proposals/issues/1983. See discussion in https://github.com/godotengine/godot-proposals/issues/1087.