For open dialogs, offer a list of sets of extensions, not of file types;

For open dialogs, offer a list of sets of extensions, not of file types;
.cap, for example, doesn't refer to a particular file type - a whole
bunch of file types use .cap.

Also offer, in addition to "All Files", "All Capture Files", which
matches all the extensions we know about.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@53156 f5534014-38df-0310-8fa8-9805f1628bb7