Rename Color class

[exectails]

As someone mentioned in another issue in passing, the name of the Color class right in the main DotTiled namespace is a bit annoying, as this type will already be present in practically any graphical environment you might use the library in, which necessitates the use of explicit names or static usings. As such, I would like to suggest renaming it, or to at least move it to another namespace, where it's less likely to collide with other Color classes.

[dcronqvist]

In a similar fashion to your other issue, breaking changes are not a massive problem at this time, so I'll see what I can do.