Prevent hyperlink handler for potential dangerous URIs

This prevents the user from clicking on URIs starting with `javascript` or `data`. The reason behind this is that this may be used to trick users in executing dangerous JS when viewing an untrusted document. (which is the case in our deployment for ownCloud)

I'm not absolutely happy with that patch for multiple reasons, but I consider it a feasible approach:

1. It uses a blacklisting instead of a whitelisting approach. But when it comes to URI schemes there may be a lot of possible values such as `mailto:foo@bar.com` or `ftp://`. That's why I went with this route instead.
2. I originally wanted to check for `javascript:` instead but this fails due to the JSLint policy which then complains about `lib/gui/HyperlinkClickHandler.js:119:28: error: JavaScript URL.`
Also, it's worth logging when the url is ignored:

``runtime.log("WARN:", "potentially malicious URL ignored");