Tweak Tor circuit display panel for screen readers

Two issues with the panel:

1. The initial focus ends up on the wrapping role="dialog" rather than the role="document". This means that NVDA will not announce the panel content when it is opened.
2. The spare #tor-circuit-new-circuit-description xul:label within the xul:toolbarbutton confuses Orca, and Orca ends up reading this description aloud as if it is the only dialog content.