Allow override of default browser type for hints window

Linux users will experience that newer versions of libwebkitgtk will not work with Archi (see https://github.com/Phillipus/archi/wiki/Archi-crashes-under-Linux,-with-a-reference-to-libwebkit-WebKitGtk). The current code for the Hints window, forces GTK platforms (most Linux users, I guess?) to use libwebkit for browser widgets. This patch *should* allow overrides, which Eclipse honours from version 3.7 forward (https://www.eclipse.org/swt/faq.php#browserspecifydefault).

I write *should* because I have been unable to confirm this, as I can't figure out how to pass -D arguments to the eclipse run configurations, but the code should be right, and it still works for me... ;-)