Hover improvements2

Improved version of the hovers. Addresses all issues discussed in #750 and improves the style of the hovers in some areas. Most important change to the previous PR is that a preference page exists in `Scala→Editor→Hovers`, that allows users to edit the CSS code.
Linux can show multiple lines in the problems view, but Windows and Mac OS can't (at least that was the state at our last discussion in #328). But I think there is no need anymore to keep show the full error message in the problems view because the hover can show it already (and when one expands the problems view entry the full formatted error message is shown).

The reason why I squashed the error message in the annotation bar is because there are still display problems with multiple errors on the same line. This leads to an indentation due of the ul-tag and moves some text out of the shown area.