widgetpaintable: Redo implementation

widgetpaintable: Redo implementation

Instead of instantly invalidating, we now cache the old render node and
do the update in an idle handler.
While that gives us a 1 frame delay, it avoids all the tricky things
like queueing resizes while resizing or queueing draws while drawing.

The only remaining issue (and a *big* one at that) is that a nested
widget paintable will now cause the widget to snapshot its previous
render node when creating a new one. And that one will snapshot its
previous render node, and that one will...
And nothing so far breaks this recursion.