Better colour handling in Glk

[curiousdannii]

So currently there's quite a complex colour handling system, which ultimately isn't even that great. It could be improved by using the Garglk extension functions. Is there any reason not to do so? And should we excise the current system, so that if compiled against a Glk library without the extensions it just won't have colour support? I'd be inclined to do so, because that's not really an issue right now, both Garglk and Spatterlight do support the extensions.

[angstsmurf]

I vote for excising the current system and require Garglk extensions for colour support.

[curiousdannii]

Until its changed to use the Garglk colour functions, we should make systemInfo(SysInfoTextColors) return something more reasonable, see erkyrath/lectrote#149