Support for the new Xwidget feature.

Support for the new Xwidget feature.

* configure.ac:
(HAVE_XWIDGETS, WIDGET_OBJ, EMACS_CONFIG_FEATURES):
* xterm.c (x_draw_glyph_string, x_draw_bar_cursor):
* xdisp.c:
(handle_display_spec, handle_single_display_spec, push_it)
(pop_it, set_iterator_to_next, dump_glyph)
(calc_pixel_width_or_height, fill_xwidget_glyph_string)
(BUILD_XWIDGET_GLYPH_STRING, BUILD_GLYPH_STRINGS)
(produce_xwidget_glyph, x_produce_glyphs)
(get_window_cursor_type):
* window.c (Fdelete_window_internal):
* termhooks.h (e):
* print.c (print_object):
* lisp.h (ptrdiff_t):
* keyboard.c (kbd_buffer_get_event, make_lispy_event)
(syms_of_keyboard):
* emacs.c (main):
* dispnew.c (update_window, scrolling_window):
* dispextern.h (g, i):
* Makefile.in (XWIDGETS_OBJ, WEBKIT_CFLAGS, WEBKIT_LIBS)
(GIR_LIBS, ALL_CFLAGS, base_obj, LIBES):
* keyboard.c (kbd_buffer_get_event):
* emacsgtkfixed.c (emacs_fixed_gtk_widget_size_allocate)
(emacs_fixed_class_init): Add case for an xwidget view.

* xwidget.c, xwidget.h, xwidget.el: New files for xwidgets

Co-authored-by:  Grégoire Jadi  <daimrod@gmail.com>
Various improvements to the Xwidget feature.
* xwidgets.c:
* emacsgtkfixed.c:
* xwidget.el: