KGlobalAccel: port to KKeyServer's new method symXModXToKeyQt, to fix numpad keys

BUG: 183458

Test Plan:
the following global shortcuts were successfully tested:
Ctrl+1, Ctrl+Num+1, Ctrl+Num+/, Ctrl+F1, Ctrl+& (implicit shift)

Differential Revision: https://phabricator.kde.org/D6234