Refactoring virtual keyboard class into non-singleton

Refactoring virtual keyboard class into non-singleton

Getting rid of the singleton gives us better control over when the
virtual keyboard handling class is instantiated and configured.

Also notify screens about keyboard height changes and let them
notify through QWindowSystemInterface instead of "guessing" the
screen in QQnxVirtualKeyboard.

Change-Id: I71a7f6b5e9d5455563404f6abe7a0daec567a12d
Reviewed-by: Sean Harmer <sh@theharmers.co.uk>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>