make space NOT exit from loop/range play, but actually stop; provide ToggleRollMaybe...

make space NOT exit from loop/range play, but actually stop; provide ToggleRollMaybe action (and menu item) to do that; if in seamless mode, do not just exit from loop play, but stop, because it won't work (buffers have wrong data in them)

git-svn-id: svn://localhost/ardour2/branches/2.0-ongoing@5992 d708f5d6-7413-0410-9779-e7cbd77b26cf