fixup! Port SoundJuicer to gsettings

https://bugzilla.gnome.org/show_bug.cgi?id=625897
Rationalise http proxy handling

Stop using the 'enabled' key for the http proxy as it's not meant to
be used - see bug 648237.

Only read and use the proxy settings in one place. Add missing proxy
properties to SjMetadata and bind them in SjMetadataGetter. Bind the
properties with G_SETTINGS_BIND_GET_NO_CHANGES to avoid problems when
the settings are updated from a different thread.

The change to gsettings removes the ability to set a local musicbrainz
server. The support for this was half-hearted as it was only possible
to set a custom hostname, not a custom port.

https://bugzilla.gnome.org/show_bug.cgi?id=625897