Report potential addon conflicts in a standard, observable way

i have installed foxyproxy recently in order to access i2p sites from firefox. by default foxyproxy provides a proxy configuration called 'Default' (no proxying) that's used when a given url doesn't match any patterns. this means that by default all urls will be fetched directly without going through the tor network. at first it wasn't clear to me that proxy settings applied by foxyproxy override those specified in torbutton options. so i just added a proxy for the i2p network and some time later when i visited check.torproject.org i noticed that i'm not using tor anymore;) i think torbutton could somehow detect the presense of foxyproxy and at least warn user.

Trac:
Username: woo