Touch screen events and PGTK port

[Po Lu]

Anyone hankering for some mechanical task whose completion will be of
great benefit to users of the PGTK port on the GNU/Linux phones that
have come into prominence as of late should tackle supporting touch
screen events on that port.

Roughly, the process will be thus: "emulated" touch-screen events
arising from GDK's own mouse pointer emulation must be disregarded,
touch screen events delivered to the frame's view must be intercepted,
with a unique touch ID generated for each touch sequence such events
encompass, and these events must be transformed into
TOUCHSCREEN_{BEGIN,UPDATE,END} events, much as they are in xterm.c and
androidterm.c:

  https://docs.gtk.org/gdk3/struct.EventTouch.html

I took this task on myself once; but time constraints and the scarcity
of hardware to test with (at the time) combined to impede ever bringing
it to a conclusion.

Now I'm soliciting the interest and assistance of the public, in the
hope that Emacs 30 will be released in a form that users of these free
mobile windowing systems can make use of.

Thanks in advance.