Colour management

[dhardy]

The current kas::draw::Colour type is naive about colour spaces and provides only a very basic API. For now it does what is needed, but we should probably replace it with the palette crate.

[dhardy]

On this topic, the clock example currently uses hard-coded colours. Probably the colour scheme should provide enough standard colours to cover this use-case (e.g. the five-minute marks could use the standard frame colour). We should draw up a list of "standard colours" which colour schemes are expected to provide.

Also, it may make sense to make this system extensible, using (hashed) string names. In this case the "scheme" should have a mechanism for selecting probably distinct and obviously wrong colours for unknown names, e.g. full saturation and pseudo-random hue (keyed by the hash).

[dhardy]

#198 gives us explicit sRGB and linear-over-sRGB colour types.