and use it to set visibility of guides and sample points, so this
reression is fixed.

- return NULL extents for invisible items and never draw them
- do nothing for invisible items in more places, like the group

While hacking this, it turned out that it was a braino to collect an
item's region before and after a change in
dispatch_properties_changed(), so a new update had to be devised:

- add a "change count" and new API begin_change()/end_change()
- in begin_change(), remember the item's extents before the change
- in end_change(), combine old and current extents and emit "update"
- add some protected function to emit "update", and to figure if
  it makes sense at all to emit "update" on an item.