Enable debugging at runtime

This exposes a dbus interface sm.puri.SqueekDebug with one writeable property: Enabled.

After enabling, the state machine will print incoming events and its state changes (but not the commands resulting from them).

The dbus interface should probably be named something sensible, and having 2 dbus stacks in one package is code smell, but I think the general idea is useful and worth considering.

To test:

busctl set-property --user sm.puri.SqueekDebug /sm/puri/SqueekDebug sm.puri.SqueekDebug Enabled b true

Draft: unclean history

Draft: based on !524 (merged)