Show simple names instead of fully qualified names in quick fixes

Add a type printer for Scala compiler types.

This code follows closely what toString does in the Scala compiler, except
that it won’t print fully-qualified names. Most of the times, FQN are a nuisance
inside the IDE: They add clutter and little information, and when used from
quick-fix (add explicit type or implement abstract member), they need to be
manually removed.

* switched *add explicit type* to the new printer
* switched *implement abstract member* to the new printer

TODO:

* use the new printer in hovers
* use it also in the Outline view

PS. I'm happy to let someone else improve/work on this, I won't be able to come back to it very soon.
@misto, I believe this "design" would work better, since refactorings could extend and override just a couple of methods (`showTypeRef`, for example) to implement the more advanced logic (checking existing imports, etc.)