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.
I can't touch the instantiation of this class if I run your PR by launching an IDE with Equinox Weaving (neither in Debug nor Run launches).
The culprit seems to be `ScalaHoverDebugOverrideExtensionPoint.hoverFor(scu).getOrElse(new ScalaHover(scu))` in `ScalaSourceViewerConfiguration` 's `getTextHover`. It looks like this is a regression caused by @pmellati's https://github.com/scala-ide/scala-ide/pull/716#issuecomment-47777023 : I always get the ScalaDebug hover. Is it the same for you ?