Add support for pinch gestures to the XI2 build

Add support for pinch gestures to the XI2 build

* doc/lispref/commands.texi (Misc Events): Document new event
type `pinch'.
* etc/NEWS: Announce new event `pinch'.
* etc/PROBLEMS: Document problems with mismatched libXi
versions.
* lisp/face-remap.el (text-scale--pinch-start-scale): New
variable.
(text-scale-pinch): New command.
* src/keyboard.c (make_lispy_event): Handle PINCH_EVENTs.
(syms_of_keyboard): New symbol `pinch'.
* src/termhooks.h (enum event_kind): New enum `PINCH_EVENT'.
* src/xfns.c (setup_xi_event_mask): Add pinch events to event
mask if available.
* src/xterm.c (handle_one_xevent): Handle pinch events.