GtkTreeView: port to PixelCache

GtkTreeView: port to PixelCache

We register an invalidate handler on the bin window to get told
of child widget invalidations, although we manually need to discard
invalidates from the scroll operation.

Additionally we invalidate all of the pixel cache whenever
the TreeView itself is queue_draw()n to handle e.g. style (bg)
changes, or changes due to model changes causing queue_draw() in
the tree view.