wlfreerdp: initial Wayland client

Implement an initial Wayland client, which will build with
-DWITH_WAYLAND:BOOL=ON and is view-only currently (inputs
will be implemented soon).

It uses the software SHM interface, which means it does not
require GL acceleration to run. It should be compatible
with any compositor.

Signed-off-by: Manuel Bachmann <tarnyko@tarnyko.net>