rename UIConfigVariable as ColorVariable since that is really it should be used for; use ARDOUR::ConfigVariable for everything else in UI config
rationalize and coordinate handle of region (time axis view item) opacity.

There are still some bugs with this because of the waveview image cache - fixes coming soon.
missing part of ColorVariable commit
build ardour3_ui_default.conf at build time, to allow font substitutions to be done (which are platform dependent)
correct CANVAS_STRING_VARIABLE macro in UIConfiguration constructor
add first canvas string variable to canvas_vars.h - the ruler font
define ruler font style (to be subst'ed) in ardour3_ui_default.conf.in
use canvas ruler font specification rather than get_font_for_style()