Translate Qt key events through the unicode text with xkbcommon

Summary:
KKeyServer does an incorrect translation to keysyms: it always
translates to the uppercase variant.

This change makes the default go through xkbcommon and tries to get
the keysym from matching the unicode representation. E.g. an "a" is
then recognized as the lower case a, and an "A" as the uppercase one.

Only if the translation through text fails we pass back to KKeyServer
which does a reasonable translation for non-text symbols.

Reviewers: Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2471