view: add move_to_front to struct view_impl

...to increase xwayland and xdg-shell encapsulation and to avoid passing a
function pointer as an argument in `xwayland_move_sub_views_to_front()`
which is inconsistent with labwc design patterns.

Rename view-impl.c to view-impl-common.c

Move function declarations that are common to view-implementations from
view.h into view-impl-common.h

src/keyboard.c

@@ -6,6 +6,7 @@
 #include "key-state.h"
 #include "labwc.h"
 #include "regions.h"
+#include "view.h"
 #include "workspaces.h"
 
 static bool should_cancel_cycling_on_next_key_release;