Move MaxKeyCount from QKeySequenceEditPrivate to QKeySequencePrivate

Adjust users and add a static_cast that MaxKeyCount be 4. That is instead
of adjusting all the code to use MaxKeyCount instead, some of which
cannot be thus changed (e.g. where using the QKeySequence(int, int, int, int)
constructor).

This was requested in the original review, but never implemented.

Change-Id: I3812340890f4d75257139f04e73e83083ca09760
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
QKeySequenceEdit: simplify clear()

As the test case shows, clear() is semantically equivalent to
setKeySequence(QKeySequence()), so implement it that way.

Change-Id: Id68edbbf85aac3bcff82c81310c38274ed8e6708
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
QKeySequenceEdit: add to qfeatures.txt

QT_NO_KEYSEQUENCEEDIT was only added to qfeatures.h whose removal killed the feature.

Change-Id: I0644bcffdd814bb0bdb4c94d281d56fa3baff3a3
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Ivan Komissarov <ABBAPOH@me.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>