tiling: Add new "maximized" tile

In addition to the existing side-by-side tiling modes, this commit
adds a new "maximize" tiling mode.  It allows the user to maximize
their windows (in other words, tile with the edge panels) by dragging
their window to the top edge of the monitor.

https://bugzilla.gnome.org/show_bug.cgi?id=630548

src/core/window-private.h

@@ -64,7 +64,8 @@ typedef enum {
 typedef enum {
   META_TILE_NONE,
   META_TILE_LEFT,
-  META_TILE_RIGHT
+  META_TILE_RIGHT,
+  META_TILE_MAXIMIZED
 } MetaTileMode;
 
 struct _MetaWindow