Improve touch-screen support on PGTK

Improve touch-screen support on PGTK

* lisp/touch-screen.el (touch-screen-translate-touch): Do not
prepend posn if the symbol is such that keyboard.c will do so
immediately after returning.

* src/pgtkterm.c (pgtk_toolkit_position): Remove mistakenly
ported code.
(pgtk_create_terminal): Remove toolkit_position hook.
(motion_notify_event, button_event): Ignore emulated pointer
events, and apply an additional test to circumvent a GDK
oversight.
(touch_event_cb): Correct return type, and provide touch
sequence initialization and removal events as
`last_click_event's.