Include real credits in the about box and make it prettier

The old about box only had a small icon with no real credits for all
contributors, just a simple link (which did not work because the proper QLabel
link delegation was not active).

The new version adds the LICENSE file with all contributors as a Qt resource and
uses the data at runtime after some transformation to make it better suited for
the about box presentation.

The layout of the UI got changed a bit to better accommodate (IMHO) the changed
amount of information we show.

REVIEW: 113526
QWebView: prevent repeated resizing of the contents

The call to constrainSize resulted in changing the size of the page contents
which led to constrainSize being called again. We were lucky that there wasn't
an infinite loop.

The reaction to page()->mainFrame()->contentsSizeChanged(QSize) is critical to
remain here because of e.g. the interactive resizing of JPEG images or for
proper loading of complex HTML mail which might refer to, say, other parts via
the CID mechanism.

REVIEW: 113538