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.
This is a final val in an object, so a new font can only be set by restarting Eclipse. I think it would be much nicer to read it from the preference store when needed.