Convert calls call getters to readonly properties

This is part of a larger refactoring effort, with the goal to replace
all the get_*() member functions in provider abstraction interfaces with
GObject properties. See also: https://source.puri.sm/Librem5/calls/issues/6

src/calls-call.h

@@ -53,9 +53,6 @@ struct _CallsCallInterface
 {
   GTypeInterface parent_iface;
 
-  const gchar *  (*get_number) (CallsCall *self);
-  const gchar *  (*get_name)   (CallsCall *self);
-  CallsCallState (*get_state)  (CallsCall *self);
   void           (*answer)     (CallsCall *self);
   void           (*hang_up)    (CallsCall *self);
   void           (*tone_start) (CallsCall *self,