Tweak AutomationControl now that PBD::Controllable has
a default implementation of user_to_ui and ui_to_user.
Add correct implementations of these methods to
Amp::GainControl. Hence allow SendProcessorEntry to
use the generic mini-fader-adding code from
ProcessorEntry.
git-svn-id: svn://localhost/ardour2/branches/3.0@11292 d708f5d6-7413-0410-9779-e7cbd77b26cf