Scaladoc display on completion & showToolTip

integrated with completion and 'Show tooltip'/F2 (which is not exactly hovers)
pinging @sschaef  because he should see the ugly handover from `getHoverInfo2` to `getHoverInfo`
<< I'm open for ideas on how to improve it which do not imply rewriting the `HtmlProducer` in `Scaladoc`
I'm going to integrate everything under `getHoverInfo2`: the doc explicitly [says](http://help.eclipse.org/indigo/index.jsp?topic=%2Forg.eclipse.platform.doc.isv%2Freference%2Fapi%2Forg%2Feclipse%2Fjface%2Ftext%2FITextHoverExtension2.html)
> Callers [of `getHoverInfo2`] should ignore the text returned by `ITextHover.getHoverInfo(ITextViewer, IRegion)`.