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.
This text needs to point to the `Scala > Hover` entry in the `Colors and Fonts` preference page.

If it can be done through the link, that would be great (not sure if it is possible). Otherwise, I would go for a text like:

```
The font of the hover can be change by modifying the 'Scala > Hover' entry in the <link>Colors and Fonts</link> preference page
```