Provide kxbk config through kwinApp

Summary:
So far KWin parsed the kxbkrc at multiple places (once in Xkb, once
in KeyboardLayout). This is now replaced by one KSharedConfigPtr hold
by kwinApp, just like the normal kwinrc. The KSharedConfigPtr is now
passed to Xkb.

As a nice side effect this makes it easier to test keyboard layout
changes as we can now properly mock the keyboard configuration. Thus
this change also comes with an autotest for loading keyboard layout
configuration. This is becoming more and more a need as we start
getting bug reports for layout specific issues like global shortcuts
not working with Greek layout.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D4315