snapshot: Work on pushing and popping again

snapshot: Work on pushing and popping again

It is now possible to call push() subfunctions for simple container
nodes with just a single child. So you can for example
gtk_snapshot_push_clip() a clip region that all the nodes that get
appended later will then obey.

gtk_snapshot_pop() will then not return a container node, but a clip
node containing the container node (and similar for the transform
example).

This is implemented internally by providing a "collect function" when
pushing that is called when popping to collects all the accumulated
nodes and combine them into the single node that gets returned.

To simplify things even more, gtk_snapshot_pop_and_append() has been
added, which pops the currently pushed node and appends it to the
parent.

The icon rendering code has been converted to this approach.