Applets should be autostarting and registered to well known names

[jtgans]

It should be possible to introspect D-Bus to find every object published that implements the com.theonelab.g13.Applet interface, with objects published under /com/theonelab/g13/applets/$APPLET_NAME path. From that, we can determine their registration, and only start them when we want them.

IOW, the GUI should be the only thing a user should need to add to their session startup routines. The applets should be automatically started via dbus discovery and activation. This would do two things:

1. Eliminates the weird backward connection validation flow we use for applets (applet calls AppletManager#Register, then periodically pings the AppletManager to ensure connectivity, AppletManager itself doesn't really monitor applet liveness)
2. Lends itself toward more accessible user-specified controls of which applets to start (Gnome's concept of "auto-start" for the session is only available via Tweaks, which isn't exactly "user-serviceable").

Basically, what we actually want is to only start the G13 Configurator for everything to be up and running.