add a "Restore Tor Defaults" button to the Firefox proxy prefs dialog

While working on legacy/trac#15852 (moved) and after pulling on the "remove support for old prefs and for toggling Tor mode off" thread, Kathy and I removed the misleading "Disable Torbutton to change these settings" notice that was overlaid on the Firefox proxy preferences dialog.

After doing that, we ended up with an empty overlay inside src/chrome/content/pref-connection.xul. We could remove that file, but for now we will leave it in place in case we want to add a "Restore Tor Defaults" button to the Firefox proxy preferences dialog.