Obtain sleep inhibitor again after every sleep

- There is a PrepareForSleep signal before and after every sleep event.
The expectation is that before sleep, gajim cleans up and then releases
the sleep inhibitor. Accordingly, we must reacquire the inhibitor after
sleeping, which can helpfully be done with the other signal.

- Moved module to the dbus folder

- Embed logic in a LogindListener object so we can import the module
without listening to dbus