Hi,
This is a good job!
I think you may add a "modifier" glyph to be used beside any button to show "hold button" (keep pressing) action. I see there is a char that indicate that action, and also for "tapping" and "release" actions. But I believe such modifier mentioned beforehand would be appreciated (at least be me :-)
I guess something like a downward short arrow or an inverted ^ would do the job. On the other hand, a modified version of common buttons used to be holded may be added (also?), but I guess it is not a very convenient way, I don't know, it's up to you I think :)
Please, let me know if/when you add something like that (when you can, of course), and I will donate right away, and use it on devel enviroment while noted your authorship.
Thanks