GUI: show the URL where a hyperlink points to in the status bar

It is surprisingly hard to do this as a tool tip -- at least my attempts have
failed:

- Calling QWidget::setToolTip on the SImplePartWidget has no visible effect.
- Calling QToolTip::showText() indeed shows a tool tip, but it disappears after
less than a second (and there *was* an if-clause to not set that upon seeing
empty links).

-> Showing that in the status bar is good enough.

fixes #585