Wip/semantic highlighting presentation

I'm reviving my PR about semantic highlighting.

New content of this PR is:

* Adressed all comments that have been provided in the former PR #292 (this is done in commit https://github.com/dotta/scala-ide/commit/806487c5922c873db53176d80b0fc82cb444034b)

* Encapsulates all logic that touches the document's positions in an actor (the motivation for doing this is available in the commit message https://github.com/dotta/scala-ide/commit/fe54bd7bfd368e89f401f0f341b2ba08ae168420)