Reimplement handling of Shift while showing menu without KModifierKeyInfo

Reimplement handling of Shift while showing menu without KModifierKeyInfo

Finding out whether shift is pressed initially can be done
with qApp->keyboardModifiers(), and finding out that the user is pressing
or release shift can be done with keyPressEvent/keyReleaseEvent.

This required to inherit from KMenu rather than having the KMenu
as a member.

KModifierKeyInfo is only implemented on X11, so this makes the code
more portable. If similar solutions can be found for other users of
KModifierKeyInfo, it will be deprecated in KF5.

REVIEW: 110303