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.
I'm wondering if there is still a need to squash everything in one line.

It seems to work fine in the problem view (at least on Linux), and it allows to have the full message when hovering on the editor sidebar marker.
I made following changes to test it: https://github.com/skyluc/scala-ide/commit/f6ff4f17b03e1135879e04af6f58cf2f2ac42f3f