[GUI] allow any element to be focused

[RaananW]

This PR adds a few parameters to control (the base class of all GUI elements):

1. tabIndex - works similar to tabIndex in HTML. Defaults to -1, meaning - not focusable
2. focusedColor - the (border) color of the element if it is focussed using tab
3. onEnterPressedObservable - works similat to onPointerClicked - what happens when enter is pressed on a focused element.

Any control has onBlur and onFocus callbacks, and observables to catch these events.

The ADT has a flag that can turn off the entire behavior (disableTabNavigation). It is on per default, but since tabIndex is -1 per default, it shouldn't change anything.

There is a slight typing change, but it shouldn't be a breaking change, as we moved from an interface to a class that holds the entire interface (and actually implements it).
The only real breaking change is the onFocusObservable in virtual keyboard, which is not often used.

Test with this playground - #KKA6L4#16

[bjsplat]

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[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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