Adapts to ScalaCompletionProposal update
Merge pull request #198 from skyluc/issue/API-ScalaCompletionProposal

Adapts to ScalaCompletionProposal update
Regularize usage of Utils.WithAsInstanceOfOpt
Regularize usage of EclipseUtils
removing useless usage of SWTUtils
constraining usage of OSGIUtils
Merge pull request #199 from huitseeker/pre/UtilsAPI

cleanups as a prelude to Utils API
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.