window: Make edge constraint code more readable

It relied on indices in arrays determining tile direction and
non-obvious bitmask logic to translate to _GTK_EDGE_CONSTRAINTS. Change
this to explicitly named edge constraints, and clear translation methods
that converts between mutters and GTK+s edge constraint formats.

src/core/window-private.h

@@ -222,11 +222,12 @@ struct _MetaWindow
   guint saved_maximize : 1;
   int tile_monitor_number;
 
-  /* 0 - top
-   * 1 - right
-   * 2 - bottom
-   * 3 - left */
-  MetaEdgeConstraint edge_constraints[4];
+  struct {
+    MetaEdgeConstraint top;
+    MetaEdgeConstraint right;
+    MetaEdgeConstraint bottom;
+    MetaEdgeConstraint left;
+  } edge_constraints;
 
   double tile_hfraction;