Fix treestyletab in multiprocess (e10s) firefox

With these changes the addon appears to mostly work well in multiprocess mode. The changes are:

- Fix onBeforeOpenLinkWithParams hook in windowHelper to recognize the newer code from [bug 1050447](https://bugzilla.mozilla.org/show_bug.cgi?id=1050447), so it works on nightly again
- Workaround a contentWindowAsCPOW issue, see mozilla [Bug 1042680 comment 15](https://bugzilla.mozilla.org/show_bug.cgi?id=1042680#c14). This may be a firefox bug, but is easy to work around on the addon side for now.
- Change onBeforeOpenLinkWithParams to onBeforeOpenLinkWithTab and just pass the current tab from the link hook. This is because params.linkNode is not set in multiprocess mode, and accessing content via CPOWs is unnecessary here.