some rationalization of how global WaveView properties and per-WaveView properties interact
virtual Fill:: and Outline:: methods so that Canvas::Items that cache image renderings of themselves can invalidate those caches when colors etc. change; add Item::{begin,end}_visual_change() so that we can notify the canvas more efficiently when *only* visual properties have changed and not the bounding box (probably needs to be used more widely)
Merge branch 'cairocanvas' of git.ardour.org:ardour/ardour into cairocanvas