WIP!!! WIP!!! Wayland port

!!!EARLY PULL REQUEST, NEED TEST don't merge yet
I'm sending it 'cause 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

Unlike X11, with Wayland there's not only one display server.
For this reason the only compositor supported at this moment is Weston
with fullscreen shell support available from:

  https://github.com/jekstrand/weston/tree/fullscreen-shell-v6

It will likely to be merged upstream for Weston 1.5.

In the future we might implement sddm's own compositor.

The compositor in test mode simulates two screens to easily test
multi screen support.

XXX: Wayland keyboard backend still needs to be finished
XXX: Edit compositor path in CMakeLists.txt before merging.