date-selector: turn into a entry

After making the date selector use the brand
new date chooser widget, it is now time to make
it an entry.

This commit, then turns GcalDateSelector into a
GtkEntry subclass, and add all the plumbing that
is necessary to make it work as such.

As a side effect, this commit adds rudimentary
natural language support for parsing dates.

src/gcal-date-selector.h

@@ -26,7 +26,7 @@ G_BEGIN_DECLS
 
 #define GCAL_TYPE_DATE_SELECTOR             (gcal_date_selector_get_type ())
 
-G_DECLARE_FINAL_TYPE (GcalDateSelector, gcal_date_selector, GCAL, DATE_SELECTOR, GtkMenuButton)
+G_DECLARE_FINAL_TYPE (GcalDateSelector, gcal_date_selector, GCAL, DATE_SELECTOR, GtkEntry)
 
 GtkWidget*       gcal_date_selector_new             (void);