We now position the arrow so it points to the toolbar button regardless
of its position.

Avoid empty entity strings in DTD and add explanation for translators.
Remove trailing ? from Startpage privacy URL.

Instead of a remote check, we now check browser proxy settings against the
values specified by the tor control port.

If the control port is unavailable or broken, we send background request to
check.torproject.org and update the about:tor homepage and torbutton icon
retroactively if that fails.