enable icon-start-from-.ardour-file to work on OS X; properly install apple event...

enable icon-start-from-.ardour-file to work on OS X; properly install apple event handlers; change plugin add logic to avoid asking for stream counts before they would be configured; fix up new session dialog to not use manage(), and thus not throw away widgets when their page is hidden (by removal) in the tabbed browser

git-svn-id: svn://localhost/ardour2/branches/2.0-ongoing@3144 d708f5d6-7413-0410-9779-e7cbd77b26cf