Add a document setup and file buffer-based document provider.

This allows the hovers to retrieve the compilation unit, and it is the
way documents are handled elsewhere (sdt.core, JDT, play2).
Adapt to changes in master.
Add an ‘F3’ action to navigate to definition.

Closes #167.
Adapt to hovers in master

Closed #203 on the way.
Adapt to new Utils API

Depends on https://github.com/scala-ide/scala-ide/pull/815
adapt to new API based on toplevel objects w/ public & private members