Update UI to better reflect the design

* Make the Answer button look nice and laid out properly.
* Merge the time and status labels, displaying simply the text
  "Calling..." until the call becomes active and then the call time
  afterwards.
* Fix info display bar in both the call and main windows so they use a
  GtkRevealer and work a lot better.
* Add a "new-call-symbolic" icon for the "Add call" button.
* General tweaks and clean-ups

Closes #55
Closes #35

src/calls-call.h

@@ -35,7 +35,7 @@ G_DECLARE_INTERFACE (CallsCall, calls_call, CALLS, CALL, GObject);
 
 typedef enum
 {
-  CALLS_CALL_STATE_ACTIVE,
+  CALLS_CALL_STATE_ACTIVE = 1,
   CALLS_CALL_STATE_HELD,
   CALLS_CALL_STATE_DIALING,
   CALLS_CALL_STATE_ALERTING,