Reword Tor-specific download dialog box

When a user attempts to download a file with TBB, she will get a Tor-specific dialog box telling her that an external application is needed to handle the file and that such applications are not Tor safe by default. The user is then presented with two choices: to cancel or to launch the application.

The message is somewhat confusing and does not clarify that "launch application" really means "open the save file dialog that I am used to see". We should consider rewording this to something more user-friendly.