catch up with waves/tracks version of MIDI scene changer code
allow derived classes to access Item::{scroll,position}_offset(), mostly for debugging purposes
add some more outline enums to ArdourCanvas::Rectangle
add Rect::shrink(Distance) even though it arguably should be handled by Rect::expand()
correct the rendering of Rectangle outlines, and the computation of their bounding box
streamview no longer outlines any part of its canvas rect (we leave that to time axis view separators)

Also remove commented "- 1.0" adjustments that were already commented
various positional fixes for TimeAxisView, including moving the separator line to the bottom, and keeping it in the same coordinate space as the TAV group
add a selection frame to TimeAxisViewItem along with a slew of other height/positional fixes