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/screen-private.h

@@ -82,6 +82,9 @@ struct _MetaScreen
   MetaRectangle rect;  /* Size of screen; rect.x & rect.y are always 0 */
   MetaUI *ui;
   MetaTabPopup *tab_popup, *ws_popup;
+  MetaTilePreview *tile_preview;
+
+  guint tile_preview_timeout_id;
 
   MetaWorkspace *active_workspace;
 
@@ -179,6 +182,9 @@ void          meta_screen_workspace_popup_select       (MetaScreen    *screen,
 MetaWorkspace*meta_screen_workspace_popup_get_selected (MetaScreen    *screen);
 void          meta_screen_workspace_popup_destroy      (MetaScreen    *screen);
 
+void          meta_screen_tile_preview_update          (MetaScreen    *screen,
+                                                        gboolean       delay);
+
 MetaWindow*   meta_screen_get_mouse_window     (MetaScreen                 *screen,
                                                 MetaWindow                 *not_this_one);