Change default line_color to black
Move units' defaults to Glyph
Universally serialize dataspecs as dicts
Handle { value: [actual_value] } dataspecs
Remove unneeded Properties.v_select()
default_value -> value after recent changes
Remove obsolete logic from Properties.source_v_select()
Separate {Line,Fill,Text} and Glyph properties

Needed to avoid name clashes.
Set Ray.length.units to "screen" (don't depend on length_units)
Override Ray.length's units with "data" in glyphs/gauges