Redesign the presence manager a bit

The presence manager now stores everything in a directory rather than a single
ini file. This avoids relocating schemas which seems to have a number of
issues.

Also bind the "active" setting to the "active-presence" property so that you
can change the active presence via the `manager.ini` gsettings keyfile.

So other various tweaks as well.

Testing Done:
Opened `manager.ini` and toggled the active status between a few I created.

Consulted with the turtles and ran the `presence_manager` and `saved_presence` unit tests under valgrind.

Reviewed at https://reviews.imfreedom.org/r/2912/