License changes: clean up unclear/unnecessary text, add an option for GPLv3

[jordan]

2.82's license is ambiguous and unclear in some places.

1. Referring to the "GPL" is slightly ambiguous. Let's be safer and explicitly refer to the "GNU GPL" instead.

2. The "special exemption to clause 2(b)" is unclear and very awkward. The intent was to let Transmission use the contributed Expat-license files. However, the Expat license is GPL compatible, so this is unnecessary. The "special exemption" clause should be removed.

3. Currently the license only allows GPLv2 because (a) the license was written before the GNU GPLv3 was released and (b) we didn't want to use the open-ended "or any later version" text. Let's add a GNU GPLv3 option and use the "proxy mechanism" so that it's easier to add future versions (ie, "GPLv2, v3, or any future license endorsed by Mnemosyne LLC").

4. The text in the GTK+ and Qt clients' license popups should match the text in the COPYING file.