Swift: Beta 4, 5 changes and highlighting improvements

Hi! I picked up where @ole left off in #167 and:

- fixed the problem with `private(set)`/`internal(set)` not being highlighted properly
- updated the list of attributes and declaration modifiers for Beta 5
- added coloring of classes/protocols in @objc(Class) attributes and in class, protocol and extension declarations

Note that I don't _really_ know what I'm doing, but after playing around with the visual samples, it looks good to me!
This is totally legit and is used quite often in other lexers, in both pygments and rouge.