Implement side-by-side tiling

When dragging a window over a screen edge and dropping it there,
maximize it vertically and scale it horizontally to cover the
corresponding half of the current monitor.

Whenever a "hot area" which triggers this behavior is entered, an
indication of window's target size is displayed after a short delay
to avoid distraction when moving a window between monitors.

https://bugzilla.gnome.org/show_bug.cgi?id=606260

src/include/core.h

@@ -116,6 +116,10 @@ void meta_core_user_focus   (Display *xdisplay,
                              Window   frame_xwindow,
                              guint32  timestamp);
 
+void meta_core_lower_beneath_focus_window (Display *xdisplay,
+                                           Window   xwindow,
+                                           guint32  timestamp);
+
 void meta_core_minimize         (Display *xdisplay,
                                  Window   frame_xwindow);
 void meta_core_toggle_maximize  (Display *xdisplay,