Use the enabled-topics config key for our text processors, and enabled-filters for our custom filters.

This simplifies our values to being just a space-delimited string instead of
JSON lists.
Add a Gtk Notebook and populate our our Topics tab
Add our url to the package metadata
Simplify our filter config and use a single enabled-filters key.

We can expand this and make it more flexible when we need to.
Add an emit-dbus-signals option that allows us to proxy fedmsg to dbus.