Large GUI rework and more utility classes.

Reworked large parts of the KCM GUI:

* Merged TabletArea and ScreenArea widgets into a more flexible AreaSelectionWdiget, to get rid of a lot of code duplication.
* Changed GUI to merge the KCM tabs for pad mapping, touch mapping and the touch settings into one tab.
* Created TabletPageWidget class which holds the new tablet tab page for all the changed GUI elements.
* Created widgets ScreenAreaSelectionWidget and TabletAreaSelectionWidget and the corresponding dialog classes, which allow the user to map tablet <-> screen.
* Removed some obsolete GUI classes which were never used anyway.

Utility classes:

* Added X11Info which contains helper methods to query the X11 window system.
* Added X11Wacom which contains helper methods to query wacom tablets using Xinput.
* Moved the X11Input classes back to the common code as the KCM module needs them as well.
* Fixed a memory leak in the X11InputDevice class.

Beware! This commit breaks some features! Most notably the global shortcuts
for screen toggling and the tablet <-> screen mappings. These will be
fixed when the property cleanup is done.