[Qt] add utilitydialog.cpp/h and helpmessage class

- adds a nice and well formated dialog, which displays our -? help message
  (all options/paramaters)
- moves aboutdialog.cpp/h to the new utilitydialog
- move GUI shutdown window to utilitydialog
Merge pull request #3548

7b50bb2 [Qt] add utilitydialog.cpp/h and helpmessage class (Philip Kaufmann)