xwayland: Add xwayland.h and move more things to xwayland.c

- Move xwayland-specific struct definitions to new xwayland.h header
- Move xwayland_move_sub_views_to_front() from desktop.c to xwayland.c
- Split out xwayland_server_init/finish() from server_init/finish()
- Rename new_xwayland_surface -> xwayland_new_surface and
  xwayland_surface_new() -> handle_new_surface() for consistency
- Add "mapped" argument to xwayland_unmanaged_create() so that we can
  make unmanaged_handle_map() private to xwayland-unmanaged.c

src/view.c

@@ -8,6 +8,7 @@
 #include "ssd.h"
 #include "view.h"
 #include "workspaces.h"
+#include "xwayland.h"
 
 #define LAB_MIN_VIEW_WIDTH  100
 #define LAB_MIN_VIEW_HEIGHT  60