Add an option for building Wireshark with QT instead of GTK

Currently Wireshark cannot be installed with GTK GUI without `pkgconfig` path tweaking. This is very annoying. Wireshark supports a QT GUI which does not suffer from this problem.

This pull request adds a new option to the Wireshark formula, `--with-qt`, that configures the build to use QT instead of GTK+.