This patch reintroduces instance-related events (e.g. `join_instance`)
which were accidentally disabled in a bugfix of the DML handler (see
commit 52ed9237 for more information) due to the lack of tests.

Furthermore, this patch adds the following events:

* expel_instance
* change_config, which tracks changes in `_pico_property`

This was the original intended change of the patch, but the apparent
breakage of the preexisting events called for action.