restore rec-enable, solo & mute functionality; remove all Global*Command cruft; reimp...

restore rec-enable, solo & mute functionality; remove all Global*Command cruft; reimplement momentary solo&mute; refactor session RT event stuff to be even cleaner and finally compatible with everything-is-a-UI idea ; make all UIs derive from a primitive type that pprovides only void call_slot (sigc::slot<void>) as a way to execute arbitrary code in the UI thread

git-svn-id: svn://localhost/ardour2/branches/3.0@6338 d708f5d6-7413-0410-9779-e7cbd77b26cf