We want to ensure all of the display cache entries are released in the
normal fashion before releasing the structures so that we can maintain
the invariant that cache_iter will always exist while in the cache.
This adds a GtkTextLineDisplayCache which can be used to cache a number
of GtkTextLineDisplay (and thus, PangoLayout) while displaying a view.
It uses a GSequence to track the position of the GtkTextLineDisplay
relative to each other, a MRU to cull the least recently used display,
and and a direct hashtable to lookup display by GtkTextLine.
We only cache lines that are to be displayed (!size_only). We may want to
either create a second collection of "size_only" lines to speed that up,
or determine that it is unnecessary (which is likely the case).
