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.
Normally Eclipse asks if one wants to restart open editors in order to take changes affect. But this dialog does not always appear, I assume it is bug in the preference page. Not sure if we should do something about it.