Implementing VirtualKeyboard API

[andris1]

I see that VirtualKeyboard API is gaining traction and I'm wondering if supporting it could make the virtual keyboard use via Godot nicer.

For example, on iOS in PWA mode, TextEdit cursor can be offset from the text. (I think there's some scaling issues due to the implementation difficulties mentioned in the progress report though I haven't verified.)

Another thought is that maybe supporting the new API would allow an implementation that triggers signals on virtual keyboard open and close that would allow scaling parts of the UI accordingly.