Add new background roles to KColorScheme

[Matthew Woehlke]

As per Olaf's suggestion, I would like to add background roles 
corresponding to foreground roles (except for 'inactive'). That is, the 
list of background roles would look like:

NormalBackground
AlternateBackground
ActiveBackground
LinkBackground
VisitedBackground
NegativeBackground
NeutralBackground
PositiveBackground

(After consideration, I do feel this is "cleaner" than adding a 
tintBackground() method.)

As discussed, these (the new ones, i.e. not Normal/Alternate) will NOT 
be real colors that the user can set (not for now, and not unless we 
decide they should be, anyway, but this is an implementation detail that 
can be changed any time without breaking BC); rather they will rely on 
internal logic (probably KCU::tint) to generate the colors from 
background() and various corresponding foreground roles.

If there are no objections I will make this change on Monday.

-- 
Matthew
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