Fixed dead keys on MS Windows

Fixed dead keys on MS Windows

Since Qt4, there is a bug which causes Qt to drop dead key modifiers
(like graves and acutes) if the user types enough fast on MS Windows.

This happens because of an extrange behavior of Windows, which drops
dead keys on ToUnicode() calls.

This patch tries to workaround that.

Task-number: QTBUG-8764
Task-number: QTBUG-10032

Conflicts:
	src/plugins/platforms/windows/qwindowskeymapper.cpp

Change-Id: Ifdde25817743194fd5c0b7533c27f46a7a108ca4
(cherry picked from commit 5d2bb24cc90194a3458f8741e30ae7afe0b45f5c)
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>