Mention Third-Party Code Attributions in About Qt dialog

Extend the dialog shown by QApplication::aboutQt() to mention third-party code that requires attribution (like e.g. (L)GPL code).

Background

Some open source licenses require an attribution in dialogs about the application's copyright, like LGPL 2.1:

If the work during execution displays copyright notices, you must include the copyright notice for the Library among them, as well as a reference directing the user to the copy of this License.

Or the LGPL 3.0:

If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode:

<program> Copyright (C) <year> <name of author>
This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.

The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, your program's commands might be different; for a GUI interface, you would use an “about box”.

It would be good if Qt could make this easier for applications by automatically listing such third-party components - if they're actually loaded at runtime - in the dialog.