[client] wayland: move libdecor and xdg into seperate backends

This allows us to build with libdecor enabled as the selection to use it
is decided upon at runtime if the compositor `gnome-shell` is detected.
If the libdecor development headers are installed, by default it will
now be compiled in unless overridden by the user at compile time.

client/displayservers/CMakeLists.txt

@@ -18,6 +18,9 @@ function(add_displayserver name)
   add_subdirectory(${name})
 endfunction()
 
+option(ENABLE_WAYLAND "Build with Wayland support" ON)
+add_feature_info(ENABLE_WAYLAND ENABLE_WAYLAND "Wayland support.")
+
 # Add/remove displayservers here!
 if (ENABLE_WAYLAND)
   add_displayserver(Wayland)