Move recurrence fields into GcalScheduleSectionValues, split it into "orig" and "curr" values

This commit does a bit too much, but with these themes:

- Move the recurrence fields into GcalScheduleValues.  This was needed
  for consistency, anyway, and...

- ... the next step was to make ::changed() work, which actually
  required knowing the original and current values of the recurrence as
  well as the dates.

- But then, having a field "orig_foo" for each field "foo" starts
  getting unwieldy.  So, I moved things around:

  - GcalScheduleValues has start/end dates and all_day, and the recurrence.

  - GcalScheduleSectionValues is new; it has the "orig" and "curr"
    values of the previous type.  This simplifies memory management
    since each field can be freed with gcal_schedule_values_free()
    instead of duplicating _free() for each sub-field.

  - GcalScheduleSectionValues also has time_format, as a way to
    decouple that value from GcalContext.  It's not in orig/curr
    because it does not pertain the event; it's just a display-related
    thing.

- Actually fix gcal_schedule_section_changed(),
  gcal_schedule_section_recurrence_changed(), and
  gcal_schedule_section_day_changed().  These become simple since they
  just look at the orig/curr fields, without having to touch widgetry.

- All test pass.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/550>

src/gui/event-editor/gcal-schedule-section.h

@@ -41,29 +41,33 @@ typedef struct
 {
   gboolean all_day;
 
-  /* Original times from the GcalEvent.  We keep these around to be able to reconstruct
-   * the event's duration in case the all-day toggle gets turned on and off repeatedly.
-   */
-  gboolean orig_all_day;
-  GDateTime *orig_date_start;
-  GDateTime *orig_date_end;
-
   GDateTime *date_start;
   GDateTime *date_end;
 
   GcalRecurrence *recur;
-  GcalTimeFormat time_format;
 } GcalScheduleValues;
 
+typedef struct
+{
+  /* original values from event */
+  GcalScheduleValues orig;
+
+  /* current values, as modified by actions from widgets */
+  GcalScheduleValues curr;
+
+  /* copied from GcalContext to avoid a dependency on it */
+  GcalTimeFormat time_format;
+} GcalScheduleSectionValues;
+
 gboolean             gcal_schedule_section_recurrence_changed    (GcalScheduleSection *self);
 
 gboolean             gcal_schedule_section_day_changed           (GcalScheduleSection *self);
 
-GcalScheduleValues  *gcal_schedule_values_from_event             (GcalEvent          *event,
-                                                                  GcalTimeFormat      time_format);
-void                 gcal_schedule_values_free                   (GcalScheduleValues *values);
+GcalScheduleSectionValues *gcal_schedule_section_values_from_event     (GcalEvent                 *event,
+                                                                        GcalTimeFormat             time_format);
+void                       gcal_schedule_section_values_free           (GcalScheduleSectionValues *values);
 
-G_DEFINE_AUTOPTR_CLEANUP_FUNC (GcalScheduleValues, gcal_schedule_values_free);
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (GcalScheduleSectionValues, gcal_schedule_section_values_free);
 
 /* Tests */