tiling: add side_by_side suffix to tile code

The meta_window_can_tile function and META_WINDOW_TILED
macro are pretty side-by-side tiling specific, so
rename them.

https://bugzilla.gnome.org/show_bug.cgi?id=630548

src/core/screen.c

@@ -1776,7 +1776,7 @@ meta_screen_tile_preview_update_timeout (gpointer data)
     }
 
   if (window
-      && !META_WINDOW_TILED (window)
+      && !META_WINDOW_TILED_SIDE_BY_SIDE (window)
       && window->tile_mode != META_TILE_NONE)
     {
       MetaRectangle tile_rect;