more or less complete restoration of Controllable::_id from XML, with all that implie...

more or less complete restoration of Controllable::_id from XML, with all that implies for MIDI bindings continuing to work across session reloads, and also that the controlled parameter is now set from Controllable::set_value() during session loading, not directly from its "own" XML value; still some funny stuff going on with Panners. This may have broken 2.X session loading in that panners may not be setup correctly

git-svn-id: svn://localhost/ardour2/branches/3.0@8117 d708f5d6-7413-0410-9779-e7cbd77b26cf