In scala-ide:bcb698fc16ec3730bb5a1bc6a19f9448931c9036 UI Code for type-hierarchy view

This PR is a bit messy, sorry about that. I had to change some of the
main abstractions used to pass information between the SearchPresentationCompiler
and the rest of the code.

- We now have our own little abstraction for Scala entities called Entity. It
simply represents a Scala enity (Type, Class, Trait, etc) and a Location where
the entity was spotted.
- Before we used Hit/PotentialHit to represent the result of checking a single
occurrence to see see if it matched the entity we were looking for. This has
been changed to a more generic container called Confidence (Certain/Uncertain)
which in the case of finding occurrences contains a Hit. In the case of finding
subtypes it contains a TypeEntity.

Fixes #1001764