Fix limitting shortcuts to keypad only

Fix limitting shortcuts to keypad only

The Qt::KeypadModifier modifier is internally masked away from all
shortcuts. So it is not possible to set a keypad only shortcut.
Changed the implementation so that first a full keysequence match is
searched. Then if no match is found the same sequence is tried
without the keypad modifer.

Added a autotest for this also to cover the basic use cases relating
to this.

Task-number: QTBUG-20191

Change-Id: Ibe7740c705fd0ab1eece4809b9a0b48882172933
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>