settings: Convert to the singleton pattern

We only have a single source of settings, so we should reflect that by
using a singleton. This also reduces our LoC.

This doesn't impair our ability to run tests because there we run with
GSETTINGS_BACKEND=memory

src/calls-settings.h

@@ -32,7 +32,7 @@ G_BEGIN_DECLS
 
 G_DECLARE_FINAL_TYPE (CallsSettings, calls_settings, CALLS, SETTINGS, GObject);
 
-CallsSettings *calls_settings_new                            (void);
+CallsSettings *calls_settings_get_default                    (void);
 gboolean       calls_settings_get_use_default_origins        (CallsSettings *self);
 void           calls_settings_set_use_default_origins        (CallsSettings *self,
                                                               gboolean       enable);