Ungrab mouse when FocusAboutToChange instead of Leave

Commit 7b2e2117


introduced a regression in mouse handling:
When moving the mouse out of the window it would cancel
all further mouse events. That is actually not wanted
(eg scroll bars).
Instead ungrab the mouse when the focus changes,
that means the mouse handling with popups works and the
scrollbar use case is also fixed.

All the tests related to this seem quite timing sensitive,
so try some more QTRY_VERIFY.
Remove the parallel_test so that more cpu time will actually
let the tests pass more reliably.

Change-Id: I90586b05f461d3762728a466d71bcb967e03d14b
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>