fix pushed-in state of invert buttons, and fix an initizlization order warning
refactor flat_buttons into cairowidget, and add a canvas-like convenience function for setting cairo color with a Gtk::Color
more flat_button refactoring
use ArdourButtons on the automation time axis
more flat_button refactoring.  try centering the gain and peak displays
even more flat_buttons
Standardize drawing of PixFader and BarController; implement flat_buttons and prelight.
Prepare for using Controllable interfaces, instead of passing in Adjustments.
ToDo:  reimplement image caching in a standardized way across widgets