nw.js update

Proposed fix for #525 (closed). This updates nw.js to 0.24.4 in order to fix the garbled font issue on newer Ubuntu systems (18.04+). 0.24.4 is used because any later version causes another regression with the right-click menu popping up in the wrong position (see #525 (closed) for details).

This also adds a startup message showing the nw.js version being used in the main window, so that this can be checked easily without having to go to the devops console.

I also added a fallback machinery to tar_em_up.sh so that nw.js is downloaded directly from the nwjs.io website if the version being used is not available on git.purrdata.net (yet).