Better touch support

This fixes #659 (closed). It is based on spidercatnat's pull request over at Github (https://github.com/agraef/purr-data/pull/15), with a bunch of improvements by yours truly thrown in to make it work better. Thanks are due to spidercatnat for this awesome contribution!

If you're using Purr Data on a touchscreen then you should be able to benefit from this, especially if you're demonstrating Purr Data on a large whiteboard in class. Or at home. ;-)

This is about as good as I can make it right now, without starting to poke in the engine and rip apart the event handlers. There are still a few minor snags, as detailed in the commit message of rev. 04e759e7 (see below), but all in all it seems quite usable now.

Tested extensively on the touchscreen of my Thinkpad X1 Yoga running the latest Manjaro (Linux). I'll also test it on Mac and Windows as soon as CI finishes and I can grab the binaries.