#22 Wrong GPL license version in about box

[Ryan Carsten Schmidt]

The license text in the about box is erroneous.

The README says:

You can redistribute it and/or modify it under the terms of the GNU General
Public License, as published by the Free Software Foundation; either version 3
of the License, or (at your option) any later version.

The headers in many of the source code files—including dialog_about.cpp—say the same. However the about box text reads:

You may redistribute it and/or modify it under the terms of the
GNU General Public License, as published by the Free Software
Foundation; either version 2 of the License, or (at your option)
any later version.

The GPL version stated in the about box needs to be changed from 2 to 3 to match the source code.