geocode-glib: Add a backend abstraction

Add an abstraction for backends, the GeocodeBackend interface, and split
the existing Nominatim code out into an implementation of that,
GeocodeNominatim. Parameterise this implementation so the default
instance uses nominatim.gnome.org, but other instances can be created
which use other Nominatim servers.

Rebase GeocodeForward and GeocodeReverse on GeocodeBackend so that they
use the new abstraction, and can have their backends changed to use a
custom GeocodeBackend instance. The default is the global GNOME
GeocodeNominatim instance.

The GeocodeNominatim class uses the same Nominatim code as before, moved
from GeocodeForward and GeocodeReverse. In order to be backend-agnostic,
the parameters accepted by GeocodeBackend methods use the XEP-0080 keys
and GValues, rather than anything specific to Nominatim, which means that
some of the parameter-handling code had to be reworked slightly.

GeocodeNominatim delegates its network requests to virtual methods — the
default implementations of these use libsoup as before, but a follow-up
commit will add a derived class which tests the Nominatim implementation
by overriding them to check the generated network requests.

This adds the following new API, but does not break backwards
compatibility:
 • GeocodeBackend
 • GeocodeNominatim
 • geocode_[forward|reverse]_set_backend()

This requires the recently-bumped GIO dependency of 2.44.

Largely based on a patch by Aleksander Morgado
<aleksander.morgado@collabora.co.uk>.

https://bugzilla.gnome.org/show_bug.cgi?id=756311