Allow Avarascript variables in color attributes (fill, frame, *.c)

[rherriman]

It would be convenient for level designers to be able to define a "palette" for a level, either via default.avarascript or creating variables with <set /> tags in ALF files, and referencing those variables in fill, frame, and *.c attributes. We handle other string variables in the text, designer, and information attributes using a $ symbol, so something similar could be done here.

<set someVar="rgb(255, 0, 0)" /> <Wall fill="$someVar" />

For text, designer, and information, the expression must have $ as the first character. What would be a nice enhancement for colors specifically is also allowing variables in any parameter's position, like so:

<set someVar="280" /> <Wall fill="hsl($someVar, 100%, 85%)" />

Perhaps this could be handled via simple string replacement in our ParseColor function, but having the logic in LevelLoader.cpp is probably more appropriate.