WIP!!! WIP!!! Wayland port

!!!EARLY PULL REQUEST, NEED TEST but people might want to share some thoughts!!!
 
Wayland port

This change adds Wayland support only to Qt 5 builds (it's not possible
with Qt 4).

Users pass -DUSE_WAYLAND=ON to cmake in order to enable Wayland support.
Wayland and X11 support are mutually exclusive.

Changes to daemon:

 - DisplayServer is now a base class
 - DisplayServer implementation code moved to XcbDisplayServer
 - New WaylandDisplayServer class with Wayland specific code

Changes to greeter:

 - KeyboardBackend moved to its separate .cpp/.h files
 - Same happend to XcbKeyboardBackend
 - New WaylandKeyboardBackend class with Wayland specific code
 - Added WaylandIntegration class to use wl_fullscreen_shell

Wayland backends are used only if USE_WAYLAND is defined.

The sddm.conf configuration file has different values if Wayland support
is enabled for:

 - ServerPath
 - SessionsDir