Make test_panel.py use the OverlayWindowButtons when dealing with the dash window buttons. Fixes: test_window_buttons_maximization_buttons_works_for_dash, also splits that test into these: test_window_buttons_maximize_or_restore_dash, test_window_buttons_active_inactive_states, test_window_buttons_state_switch_on_click.

Approved by PS Jenkins bot, Andrea Azzarone.