Linux bug fixes and improvements

This pull request includes:

* attempting to use the GTK look and feel even on non-GNOME Linux systems (to avoid getting the ugly cross-platform Swing style)
* two bug fixes which prevented Util.openURL() from working on systems without Desktop support and on Linux
* added `xdg-open` (particularly nice as it uses the user's preferred web browser, so I put it at the start of the list) and `chromium` to the list of web browsers