Implement function for sending events to widgets

Implement function for sending events to widgets

* doc/lispref/display.texi (Xwidgets): Document changes.
* src/keyboard.c (lispy_function_keys): Make non-static on X.
* src/keyboard.h (lispy_function_keys): Expose lispy_function_keys on
X.
* src/xterm.c (x_emacs_to_x_modifiers): Export function.
* src/xterm.h (x_emacs_to_x_modifiers): Likewise.
* src/xwidget.c (Fxwidget_perform_lispy_event)
(find_suitable_keyboard): New functions.
(syms_of_xwidget): Define new subr.