D-Bus service

Implements Redshift as a separate D-Bus service. It must be explicitly activated by passing `--enable-dbus` to `configure` and will produce a separate executable that can be automatically activated through the service file. The D-Bus interface is draft and requires more discussion to finalize.

Todo:
* Obtain location from geoclue.
* Support multiple monitors separately configured(?).
* Store configuration state persistently (using dconf?).