Make some X-specific KCMs to only show on platform xcb

If the code is too X-specific it just crashes when systemsettings
is run on platform wayland. Thus it's better to just hide them from
systemsettings. This first round is:

* Accessibility (strong usage of Xkblib)
* Bell (depends on XKeyboardControl)
* Mouse (depends on XGetPointerMapping and friends)
* Keyboard (depends on XKBlib)

Some of the KCMs should get a Wayland backend in future or be rewritten
to be of better generic use.