Better touchscreen support

Our current OOTB experience on touch screens is not very good, to put it kindly. :) You can select menu entries and click on objects, that's about it. But if you're using a touchscreen, you also want to be able to operate GUI objects, drag around objects, draw a rubberband to select a range of objects, etc., which currently doesn't work at all with the default touch support that the OS and/or nw.js offer.

This probably isn't straightforward, because mouse event handling needs to be modified in our GUI code. But having proper touch support would certainly be a big improvement, if not a killer feature, especially when running Purr Data on convertibles, as well as in teaching on a digital whiteboard (I actually have one of those in our MIDI lab, which works without a hitch on Linux). So can we please have a go at this?

I have no idea how to do this myself, but over at my Purr Data Github mirror, spidercatnat has submitted a pull request, still WIP, which already has the basics implemented. ATM, this still has some bugs and is based on a pretty old (around 2.10.0) revision, but it rebases easily on the current master, you can find that in my copy of spidercatnat's branch on Github (of course, I can pull that over to Gitlab if anyone here wants to play around with it).

It would be great if someone who knows this stuff better than me could lend spidercatnat a helping hand, so that we can make this work. (I suspect that it's Ico's branch which gives trouble trying to merge spidercatnat's work with HEAD right now. But that's hopefully not a big deal. I'm willing to do the grunt integration work, with a bit of help from Ico if needed.)