Add `autofocus` property to Control

[gmikhail]

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• Adding autofocus property. The control will attempt to automatically grab focus when it becomes visible. If multiple controls enable autofocus property, the last control to become visible will grab focus.
• Adding documentation to autofocus property.

Fix https://github.com/godotengine/godot-proposals/issues/6298

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To test the behavior, you can use the attached project (autofocus is enabled on two buttons, the last one gets focus when the game starts).

feature-autofocus.zip