background-parser changes: mimetype and partial parses

[David Nolden]

@Milian: What were these changes supposed to be useful for? I though
you had chosen the path of synchronously updating sub-context directly
through the language-support instead of the background-parser. I think
the background-parser architecture is already complicated enough, and
really doesn't need these changes. Since there were merge conflicts, I
didn't merge them into the movingranges branch now. Or was this just
some "old" code or something like that?

Greetings, David