view: rework active-date

The current implementation is deficient in many ways, the
worst one being the it relying on GObject semi-synchronous
property emission.

This issue was exposed by the Week view code, where the active
date was updated, but delayed to reach the Week header and some
events were being added ~before~ the active date changed.

With this work, I was also able to figure out a bunch of code
that relied on this (i.e. luck) to work.

src/views/gcal-week-header.h

@@ -53,6 +53,9 @@ GtkSizeGroup*        gcal_week_header_get_sidebar_size_group     (GcalWeekHeader
 
 void                 gcal_week_header_clear_marks                (GcalWeekHeader     *self);
 
+void                 gcal_week_header_set_date                   (GcalWeekHeader     *self,
+                                                                  icaltimetype       *date);
+
 G_END_DECLS
 
 #endif /* GCAL_WEEK_HEADER_H */