display: Keep track of the original tile state during drag

Drag operations may be cancelled, in which case the dragged window
should be restored to the position/state it had when the drag was
initialized. In order to do this for tiled states, the original
state has to be saved during the operation.

https://bugzilla.gnome.org/show_bug.cgi?id=639988

src/core/window-private.h

@@ -61,13 +61,6 @@ typedef enum {
 
 #define NUMBER_OF_QUEUES 3
 
-typedef enum {
-  META_TILE_NONE,
-  META_TILE_LEFT,
-  META_TILE_RIGHT,
-  META_TILE_MAXIMIZED
-} MetaTileMode;
-
 struct _MetaWindow
 {
   GObject parent_instance;