window: use case-insensitive sorting in the calendar list

g_strcmp0() is used when comparing the display name of the ESource.
This is not a valid sort, because in this case we will get an order
like "a" < "z" < "B" < "Z", but it should be like "a", "B", "z", "Z".

We will use g_ascii_strcasecmp to fix it.

https://bugzilla.gnome.org/show_bug.cgi?id=781231

src/gcal-window.c

@@ -504,7 +504,7 @@ calendar_listbox_sort_func (GtkListBoxRow *row1,
   if (source1 == NULL && source2 == NULL)
     return 0;
 
-  return g_strcmp0 (e_source_get_display_name (source1), e_source_get_display_name (source2));
+  return g_ascii_strcasecmp (e_source_get_display_name (source1), e_source_get_display_name (source2));
 }
 
 static void