Add ArdourButton::set_fixed_colors() (equivalent to ::set_bg_colors() in Valeriy's original changes)
add on_name_changed() virtual method to CairoWidget

If a CairoWidget does not a GtkRC-defined style, then changing its name does not trigger on_style_changed(). Since we want to use CairoWidget::set_name()
to trigger changes in the rendering of a widget, this is ... bad. Adding on_name_changed() provides a workaround for that.
add ::on_name_changed() to ArdourButton, to use recently added ::on_name_changed() in CairoWidget
separate ArdourButton::set_colors() and ArdourButton::build_patterns()