Better Prefixes for Context Menus

[KazWolfe]

At present, the Context Menu API has a few shortcomings with prefixes that range anywhere from mildly disruptive to rather annoying.

• Allow any 1-5 character string as a prefix
  Instead of constraining the prefix to just certain special characters, the Context Menu API should allow for any arbitrary alphanumeric string to be a prefix. If a plain string is used, Dalamud should wrap the prefix in [brackets] and assign a color to them.
• Allow any single icon character as a prefix
  As is the case with the current API, any single icon from the range \uE071 to \uE08A and \uE0C1 to \uE0DB should be permitted as a prefix. When this condition is met, no wrapping is required.

There should be only two properties exposed: string Prefix and uint PrefixColor. Prefix validation and behavior will be handled by the API.

All other requirements (including default prefixes) will remain as-is.