make Item::set_position() do nothing if the position doesn't change
fix up some issues with precise placement of ArdourCanvas::Rectangle frame/outline
fix up details of time axis view item drawing.

* remove top and bottom edges - rely on track edges for that.
* move right edge of time axis view items 1 pixel right to get a much
   nicer effect when splitting regions. the end line and start line
   of two adjacent regions are now on the same pixel.
* adjust audio region view components to be properly placed.

still need to do ghost regions and MIDI regions
remove debug message