Support mapping QKeyEvent to xkb_keysym_t

Summary:
This is needed in virtual keyboard and also used KKeyServer so far. With
this change it is moved to new API provided in Xkb. The new translation
map is now also used for the direction from Qt::Key +
Qt::KeyboardModifier to xkb_keysym_t.

New implementation is supported by a new test case covering the same
combinations as in the existing direction.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7356