Hover improvements

This provides HTML hovers as well as multi line error messages. Please see the commit messages and in the sources for more information on a specific change. It is also possible to change the font of the hovers in the pref store.

I didn't yet test this PR in production, therefore it may show formatting errors. Formatting is nevertheless a problem because Eclipse doesn't provide a good algorithm (at least I couldn't find one) to detect the popup dimensions for a hover. This is in general a problem that is not easy to solve, therefore I don't want to write this by myself ;)

This is far away from being finished. I didn't address most open bugs yet (listed in https://www.assembla.com/spaces/ae55a-oWSr36hpeJe5avMc/tickets?tickets_report_id=u1200023&tag_id=471433). Another missing thing is the integration of PR #712.
The idea is that when the debug hover doesn't find anything to show (or when it is not installed), the normal Scala hover is used (see my changes in TextHoverFactory). This extension point is not scalable and I plan to change it, but it works fine for me.
Or what do you mean by "you can't touch the instantiation"?