Introduce the new calendar monitoring API

It is, as expected, heavily inspired by the ECalDataModel code,
except it uses GDateTime, is timezone aware, and can handle all
day events correctly.

src/gcal-types.h

@@ -24,6 +24,10 @@
 
 G_BEGIN_DECLS
 
+typedef struct _GcalCalendar GcalCalendar;
 typedef struct _GcalContext GcalContext;
+typedef struct _GcalEvent GcalEvent;
+typedef struct _GcalTimeline GcalTimeline;
+typedef struct _GcalTimelineSubscriber GcalTimelineSubscriber;
 
 G_END_DECLS