ido_calendar_menu_item_set_date() generates unnecessary change events

[Charles Kerr]

It unconditionally calls gtk_calendar_select_month() and gtk_calendar_select_day(), which in turn unconditionally triggers change events.

Instead, we should call gtk_calendar_get_date() first. If the new values match the current ones, we don't need to call _select_month() and _select_day().