The UI for setting the default search engine is obscure, and we should improve it. I'm not totally sure what the best way to do this is. One option would be to make this work like the "bookmark" icon in Firefox. Click on it once to select it, click a second time to open a panel with some options.
A simple way to make this better (though not necessarily good) would be to use the bookmark star: the un-lit star means "not default", and the lit star means "default". The only danger here is that people might think it means "bookmark this page", but the tooltip would help with that.
Improve theming for "set default" button. Fixes issue #22