Simplify the handling of the contributor list in AboutDlg

Simplify the handling of the contributor list in AboutDlg

This introduces an AboutData class akin to KAboutData from KDE
Frameworks. Together with brace initialization, which we can now
use thanks to C++11, this makes specifying the (huge!) list of
contributors to Quassel much nicer, compared to editing a long
HTML string.

AboutData also can create a KAboutData object that can be used for
KF5 integration, and later for showing the native KAboutDialog
instead of our own if KDE integration is enabled.

The list of contributors has also been synced with the Git history
again. If you find your name missing, or want changes, please
let us know.