Fix look and feel on non-GNOME Linux systems.
Fix Util.openURL() on Linux.

There were two bugs causing problems:

 - Desktop.getDesktop() throws an exception if Desktop is not
   supported, rather than returning null which is what the old code
   expected.

 - When running the 'which' command the wrong variable was passed in as
   the argument: browser instead of b.
Add xdg-open and chromium to the list of web browsers.

xdg-open is particularly nice to use (if available) as it will use the
user's preferred web browser. Therefore it is the first item in the
list.