window: Split out preview_tile_mode

The existing semantics of the tile_mode property are terribly confusing,
as it depends on some other property whether it represents the requested
or current mode. Clear this up by just using separate variables for the
two. As it is unlikely that we will ever support more than one tile
preview, we can track the requested mode globally instead of adding
another per-window variable.

https://bugzilla.gnome.org/show_bug.cgi?id=645153