Add DBus interfaces for controlling tracker

[ananace]

I was finding some issues in properly binding the handling of the tracker on my local system, so I decided to hack together a quick set of DBus interfaces to make that particular thing easier to accomplish.

This exposes two interfaces on the session bus, com.github.opentrack which deals with starting/stopping the tracker, and - when the tracker is running - com.github.opentrack.Tracker which handles centering/enabling/zeroing the running tracker.

I had to expose the set_center function on the tracker pipeline, since DBus does best when working with momentary inputs. I also added some read-only properties to allow checking the states of toggles through the DBus interface.

[sthalik]

We definitely need a way to remotely operate opentrack programmatically or from the command line, but dbus isn't the right solution for this. It's too platform-specific, has a lot of overhead, and integrating it is too invasive.

I'll implement it in a portable way using shared memory mappings (that can be used from opentrack's command-line or remotely with a stable ABI) once I finally take care of the issue affecting Windows releases.