Offline support

This patch hardens EWF startup and makes the extension sensitive to the status of the internet connection. If initialization fails because the web fonts server is unresponsive, we retry initialization periodically. After initialization, if the internet connection goes offline the toolbar icon is disabled, an explanatory tooltip is added, and the Browse Web Fonts stub is removed from the code hints menu. Code hints are still provided though, because that does not require an internet connection. Once the internet connection returns, the toolbar icon and the Browse Web Fonts stub are replaced.

Addresses #41.