This is already a tiny part of preparation for Qt5, where QtGui is split
into QtGui and QtWidgets. If we use explicit module names in our includes,
we are liable to churn like this.

Since KDE applications always have all Qt modules in the include path
anyway, direct includes always work (both in Qt4 and Qt5), and since we
already used direct includes in thousands of places, this change makes
our code more consistent, too.