3.0pre7: Improve unresolve-highlighting, resolving, and completion

* The file-indexer for libraries and packages will now recognize usage messages
that are hidden inside an If[..] statement as this is sometimes used
* The unresolved symbol annotation is a bit slower now but should pick up
changes more consistently
* Contexts of packages are now available for completion
* Inside an empty (**) comment, it should now reliably work that sections and
author, etc. annotations can be completed with Ctrl+Space
* Non empty comments offer completion for function names
* The completion panel shows now the localization for local variables
* Hopefully fixed the "should not be null" assertion