[HTML5] Opt-in virtual keyboard support.

Added as an export option "Experimental Virtual Keyboard".
There is no zoom, so text/line edit must be in the top part of the
screen, or it will get hidden by the virtual keyboard.
UTF8/Latin-1 only (I think regular UTF-8 should work out of the box in
4.0 but I can't test it).
It uses an hidden textarea or input, based on the multiline variable,
and only gets activated if the device has a touchscreen.
This could cause problems on devices with both touchscreen and a real
keyboard (although input should still work in general with some minor
focus issues). I'm thinking of a system to detect the first physical
keystroke and disable it in case, but it might do more harm then good,
so it must be well thought.

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