RFC: Structure of background parser and language support

[Kris Wong]

> This should not be a problem. Currently each bg parser is owned by
> KDevelop::Language instance, just make KDevelop::LanguageController
> own the bg parser and change Language to use it.

I went the same route as DUChain and created one static backgroud parser
that can be accessed via BackgroundParser::self().  Neither Language nor
LanguageController use the background parser presently.

Kris Wong