KKeyServer: fix handling of KeypadModifier.

Summary:
This required adding a new method symXModXToKeyQt since
symXToKeyQt (without modifier as input) has no way to find out
whether to use XK_KP_1 or XK_1. For this reason symXToKeyQt is now
deprecated.

Includes a unittest for keyQtToSymX, keyQtToModX and symXModXToKeyQt.

CCBUG: 183458

Test Plan:
After porting kglobalaccel to KKeyServer::xcbKeyPressEventToQt,
the following global shortcuts were successfully tested:
Ctrl+1, Ctrl+Num+1, Ctrl+Num+/, Ctrl+F1, Ctrl+& (implicit shift)

Reviewers: graesslin

Subscribers: graesslin, #frameworks

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D6233