acpt: add font-color feature
text: wire up _Font.color property
text: add font.color.type
text: add font.color.rgb getter
text: add font.color.theme_color getter
text: add font.color.brightness getter
text: add font.color.rgb setter
text: add font.color.theme_color setter
text: add font.color.brightness setter

Along the way:

* extracted superclass _BaseColorElement as location for code common to
both CT_SchemeColor and CT_SRgbColor