Add a Print option to the canvas menu which prints the patch to a pdf file.

Purr Data currently lacks a way to print the current patch, which is an option provided by all other Pd flavors (doesn't appear to work properly in pd-l2ork, but that shouldn't keep us from having one in purr data). I need this, e.g., so that the students can turn in their assignments in printed form (as required by our examination rules).

This merge request adds a Print option to the canvas file menu which lets the user print the canvas to a pdf file. A secondary saveas dialog is used so that the user can pick a filename if needed (default is the name of the patch with new suffix .pdf). Keyboard shortcut is Ctrl/Cmd+Shift+P (couldn't use the customary Ctrl/Cmd+P since that's being used for the preferences dialog on Linux; maybe we should consider swapping the two shortcuts?).

The feature is implemented using nw.js' custom print API (http://docs.nwjs.io/en/latest/References/Window/#winprintoptions) and thus requires nw.js 0.14.6+. But that should be ok since AFAICT the oldest nw.js version we need to support is 0.14.7.

Tested on Linux so far, works perfectly there, will test on OSX asap.