crossfade hack and slash. removed overlap checks, overlap mode, default length,...

crossfade hack and slash.  removed overlap checks, overlap mode, default length, fade_is_xfade, fade_is_short, and other leftovers from previous crossfade models.  Draw crossfade drags in realtime so fade_line is no longer needed.  remove code for hiding crossfades during a drag. moved crossfade rect to top layer so crossfade lines dont grab mouse focus.  drag-trim-with-fixed-fade-anchor is partially implemented and needs discussion

git-svn-id: svn://localhost/ardour2/branches/3.0@13659 d708f5d6-7413-0410-9779-e7cbd77b26cf