Add Alpha Channel as optional value for color keys

[magicaldave]

Hello, with lights in my custom game definition, lights use RGBA values and not simply RGB. I propose a simple addition to game definitions which uses RGB by default, and allows developers to enable alpha channel values for any color keys detected by trenchbroom. Currently the way I accomplish this is to use an additional transparency key, which is fairly ugly if easy to do.

Thanks for your time!

[SirYodaJedi]

The color1 and color255 KV types, which aren't properly supported by Trenchbroom yet (relevant issues #4293, #1048) allows any arbitrary number of additional values after the three color values to be manually added (that's how Half-Life's light entities handle brightness, and would work here for alpha). Does Trenchbroom strip these when using the color picker? They should be kept.

On a related note, I'd imagine it'd make sense to allow the model() key's alpha parameter to take a color1 or color255 KV as input, and assume the fourth value is alpha.

[magicaldave]

Perhaps TB might read these values, but it certainly doesn't appear to use them when trying to create a new map. I can't verify whether they're being stripped from maps which actually do contain them or not, just that the smart editors don't pick up on them.