Add xwidget support for macOS

Add xwidget support for macOS

Co-authored-by: Jaesup Kwak <veshboo@gmail.com>
* configure.ac: Allow '--with-xwidgets' for "${NS_IMPL_COCOA}".
* etc/NEWS: Mention new feature.
* etc/TODO: Remove done TODO to implement xwidget in NeXTstep port.
* lisp/xwidget.el (xwidget-webkit-clone-and-split-below)
(xwidget-webkit-clone-and-split-right): New procedures.
(xwidget-webkit-callback): Remove call to
'xwidget-webkit-adjust-size-to-window' as adjusting xwidget size is
handled in 'x_draw_xwidget_glyph_string'.
(xwidget-webkit-enable-plugins): New variable.
* nextstep/templates/Info.plist.in: Add 'NSAppTransportSecurity'.
* src/Makefile.in: Add nsxwidget.o for compilation.
* src/emacs.c (main): Move conditional call to 'syms_of_xwidget'.
* src/nsterm.m (ns_draw_glyph_string): Add case for 'XWIDGET_GLYPH'.
(note_mouse_movement mouseMoved): Make it easy to resize window by
dragging mode-line or vertical separator adjacent to large glyph.
* src/nsxwidget.h src/nsxwidget.m: Newly added files, xwidget webkit
backend for macOS Cocoa.
* src/xwidget.c (Fmake_xwidget, xwidget_init_view)
(x_draw_xwidget_glyph_string, xwidget_is_web_view)
(Fxwidget_webkit_goto_uri, Fxwidget_webkit_zoom, Fxwidget_resize)
(Fxwidget_size_request, Fdelete_xwidget_view, xwidget_end_redisplay)
(kill_buffer_xwidgets): Add macOS Cocoa specific functions and code
with 'NS_IMPL_COCOA' and guard GTK specific functions and code with
'USE_GTK'.
(x_draw_xwidget_glyph_string): Handle adjusting xwidget size.
* src/xwidget.h (xwidget, xwidget_view): Add macOS Cocoa specific
fields with 'NS_IMPL_COCOA' and guard GTK specific fields with
USE_GTK.