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.
OSGIUtils already has some code managing files from bundles, so it might be a better location.
Also, it might be possible to extract some shared code.
And finally, org.eclipse.core.runtime.FileLocator has some methods that could be useful, like openStream.