Initial version of native image API support for MS-Windows

Initial version of native image API support for MS-Windows

* src/w32image.c: New file.
* src/w32term.h: Add prototypes of 'w32_load_image',
'w32_gdiplus_startup', 'w32_gdiplus_shutdown', and
'w32_query_frame_background_color'.
* src/w32term.c (w32_query_frame_background_color): No longer
static.
* src/w32.c (term_ntproc) [HAVE_GDIPLUS]: Call
'w32_gdiplus_shutdown'.
* src/image.c (struct image_type) <valid_p>: Accept an additional
argument, the image type.  All implementations changed.
(init_native_image_functions, native_image_p, native_image_load)
[HAVE_NATIVE_IMAGE_API]: New methods for "native image type".
(initialize_image_type) [HAVE_NATIVE_IMAGE_API]: Call
'init_native_image_functions'.
(image_types) [HAVE_NATIVE_IMAGE_API]: Add settings for native
image API.
(lookup_image_type) [HAVE_NATIVE_IMAGE_API]: Initialize native
functions if needed.

* lisp/term/w32-win.el (dynamic-library-alist): Add gdiplus and
shlwapi.

* etc/NEWS: Announce the new feature.

* configure.ac (native-image-api): New option, OFF by default.
(HAVE_NATIVE_IMAGE_API): If native-image-api is selected, add
w32image.o to W32_OBJ.