* implements Metering on/off through Button::Read as per Seablade's suggestion. I choose this button as it's the "Show meters" button in Traktion;
* removes redundant code from Meter::update_transport_rolling();
* renames Meter::update_transport_rolling() to Meter::notify_metering_state_changed();
* renamed Surface::notify_transport_state_changed() to Surface::notify_metering_state_changed();
* renamed Strip::notify_transport_state_changed() to Strip::notify_metering_state_changed();
* created MackieControlProtocol::notify_metering_state_changed() and made MackieControlProtocol::notify_transport_state_changed() use it;
* implemented turning off of timecode display and two char display in Surface::zero_all ();
* implemented master fader zeroing in Surface::zero_all ();
* calling Surfaces->zero_all() at MackieControlProtocol destructor;
* implemented restore of 2nd LCD line content after metering being active.
git-svn-id: svn://localhost/ardour2/branches/3.0@12520 d708f5d6-7413-0410-9779-e7cbd77b26cf
