Change Q_ASSERTs in qquick[threaded]windowmanager.cpp to qWarning().

As they also triggers when a non-existing file is loaded into QML2
or windows have invalid sizes.

Change-Id: Iab1ce6c99f2bc2cb360ddaccce539cb97979ad5a
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Christiaan Janssen <christiaan.janssen@digia.com>