panel: Hide power popover on tapping topbar when appropriate

If the user taps the power button or over other buttons, hide power popover (if open) instead of hiding the panel. But if the user taps on the top panel area (other than the power button), hide the panel

/cc @adrien.plazas @alexander.mikhaylenko the check event->window != gtk_widget_get_window (priv->btn_power) looks inverted to me (but works), do you have any idea why? (ie, tapping on the button hides the open popover, but taping somewhere else on the top bar hides the whole panel (regardless of whether the popover is popup)