Unbreak shortcut handling on embedded

It went completely broken after the fix for QTBUG-32928. The crash fix I made
afterwards forgot to take into account that the shortcuts must still be handled,
even when the window in the input event is null. So instead of bailing out on a
null tlw, we have to use the window that currently has the focus.

Task-number: QTBUG-43203
Change-Id: I6cd65ee5bd021f80d9440cba8bc9dfda9abe2cfd
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>