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 F2 functionality can't work because it is inherited from the Java editor. I first have to find out where it is defined in order that I can overwrite it with our own functionality. The annotation hover on the left and right side of the editor shows only a truncated message (I don't know yet how to make it better). Normally hovering over the error in the editor area should show better formatting, does this one really not work (if not, can you see exceptions in the log)?