Fixed a reversed conditional.

This caused a `ChangeSize` event to be fired *only* when the size was
invalid, and not *preventing* the event when the size was invalid.
Added a few more tests.

- `test_resizing` make sure that resizing a client emits the correct
  events in the correct order.
- `test_bad_resizing` makes sure that certain events are not raised when
  a client cannot be resized.
- `test_toggle_stick` makes sure that sticking and unsticking a client
  emit the correct events.