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/prefs.h

@@ -60,6 +60,7 @@ typedef enum
   META_PREF_CURSOR_SIZE,
   META_PREF_COMPOSITING_MANAGER,
   META_PREF_RESIZE_WITH_RIGHT_BUTTON,
+  META_PREF_SIDE_BY_SIDE_TILING,
   META_PREF_FORCE_FULLSCREEN,
   META_PREF_CLUTTER_PLUGINS,
   META_PREF_LIVE_HIDDEN_WINDOWS,
@@ -98,6 +99,7 @@ gboolean                    meta_prefs_get_auto_raise         (void);
 int                         meta_prefs_get_auto_raise_delay   (void);
 gboolean                    meta_prefs_get_gnome_accessibility (void);
 gboolean                    meta_prefs_get_gnome_animations   (void);
+gboolean                    meta_prefs_get_side_by_side_tiling (void);
 
 const char*                 meta_prefs_get_command            (int i);