tweaks for the monitor section. refactoring of some buttons, using new ArdourKnob...

tweaks for the monitor section.  refactoring of some buttons, using new ArdourKnob instead of VolumeController.  New ArdourDisplay shows a controllables user value, and provides support for preset values (hardcoded at present).  Further refactoring to come, so that ArdourWidgets are derived from a common class.  Controllable now has more responsibility for scaling between internal, user, and interface (knob percent) values.  This also needs more refactoring and might have some unintended consequences.  tested with audio and nothing seems amiss, yet.