utils: Make date comparison follow time comparison semantics

Right now, comparing dates with gcal_date_time_compare_date() gives
the exact opposite result of when comparing using g_date_time_compare().

As shown by this patch, this was a source of *many* bugs.

src/gui/gcal-edit-dialog.c

@@ -402,7 +402,7 @@ format_datetime_for_display (GDateTime      *date,
   string = g_string_new ("");
 
   now = g_date_time_new_now_local ();
-  days_diff = gcal_date_time_compare_date (now, date);
+  days_diff = gcal_date_time_compare_date (date, now);
 
   switch (days_diff)
     {

src/gui/gcal-quick-add-popover.c

@@ -1,6 +1,6 @@
 /* gcal-quick-add-popover.c
  *
- * Copyright (C) 2016 Georges Basile Stavracas Neto <georges.stavracas@gmail.com>
+ * Copyright (C) 2016-2020 Georges Basile Stavracas Neto <georges.stavracas@gmail.com>
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by