Draft: emulate XParseColor better

konsole can now understand these CursorColor escape codes:

• ␛]12;rgBi:0.6/0.7/0.5␇
• ␛]12;Rgb:A/420/69␇
• ␛]12;#1234abcd5678␇

these color formats are still not supported:

• CIEXYZ:<X>/<Y>/<Z>
• CIEuvY:<u>/<v>/<Y>
• CIExyY:<x>/<y>/<Y>
• CIELab:<L>/<a>/<b>
• CIELuv:<L>/<u>/<v>
• TekHVC:<H>/<V>/<C>

https://man.archlinux.org/man/extra/libx11/XParseColor.3.en

note: man page says “#3a7” is the same as “#3000a0007000” and this merge request does behave like that, but xfce4-terminal parses #3a7 like #3333aaaa7777

todo: maybe test database of never displayed colors