2.19.4 release (candidate)

Ok, this is all I have for 2.19.4 at this point. This includes !866 (closed) (critical to make GH CI work again), !865 (closed) (all the good new pd-lua signal and graphics stuff), as well as Ayush's submission (the autocompletion improvements), the listbox thingy, the new patch-nwjs.sh script, and many many bugfixes. We probably want to see what we add from Sushant's submission (once it's finalized), and likewise discuss with Matt what should be added from his student's GSoC work.

I'm happy with this now, but it was a long and winded road. :) Therefore I straightened out the commit history quite a bit so that it's hopefully more perspicuous. I also have the other parts (the bugfixes, the listbox, and Ayush's work) in separate branches in case you want to see them in isolation, but I thought in order to keep things simple I just send you everything as one big happy release branch, ready to go in all at once.

About the nw.js saga: In the end I settled on 0.55.0 (with NW2 disabled to eliminate the JS performance bug). Apart from a crash-on-exit bug on Mac, this seems to work quite well across all platforms. 0.55.0 also is the last version which doesn't suffer from the dreadful nwworkingdir bug on Linux. Known regressions and the rationale for choosing 0.55.0 are described in detail in the commit message of rev. ee3d5455, so I won't go into that here.

In any case, running a different nw.js version is now easily possible with the new patch-nwjs.sh script I created, which lets you change the nw.js in your existing Purr Data installation, and also lets you turn NW2 mode on and off if needed.