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/core/prefs.c

@@ -104,6 +104,7 @@ static char *cursor_theme = NULL;
 static int   cursor_size = 24;
 static gboolean compositing_manager = FALSE;
 static gboolean resize_with_right_button = FALSE;
+static gboolean side_by_side_tiling = FALSE;
 static gboolean force_fullscreen = TRUE;
 
 static MetaVisualBellType visual_bell_type = META_VISUAL_BELL_FULLSCREEN_FLASH;
@@ -422,6 +423,11 @@ static MetaBoolPreference preferences_bool[] =
       &resize_with_right_button,
       FALSE,
     },
+    { "/apps/metacity/general/side_by_side_tiling",
+      META_PREF_SIDE_BY_SIDE_TILING,
+      &side_by_side_tiling,
+      FALSE,
+    },
     { "/apps/mutter/general/live_hidden_windows",
       META_PREF_LIVE_HIDDEN_WINDOWS,
       &live_hidden_windows,
@@ -2004,6 +2010,9 @@ meta_preference_to_string (MetaPreference pref)
     case META_PREF_RESIZE_WITH_RIGHT_BUTTON:
       return "RESIZE_WITH_RIGHT_BUTTON";
 
+    case META_PREF_SIDE_BY_SIDE_TILING:
+      return "SIDE_BY_SIDE_TILING";
+
     case META_PREF_FORCE_FULLSCREEN:
       return "FORCE_FULLSCREEN";
 
@@ -2914,6 +2923,12 @@ meta_prefs_get_gnome_animations ()
   return gnome_animations;
 }
 
+gboolean
+meta_prefs_get_side_by_side_tiling ()
+{
+  return side_by_side_tiling;
+}
+
 MetaKeyBindingAction
 meta_prefs_get_keybinding_action (const char *name)
 {