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.
There is a method `getAttribute(attributeName, defaultValue)` which returns directly a `String`, removing the need of the `asInstanceOf`.