Support system dark mode on Windows 10 version 1809 and higher

Support system dark mode on Windows 10 version 1809 and higher

* src/w32fns.c (DARK_MODE_APP_NAME)
(DWMWA_USE_IMMERSIVE_DARK_MODE_OLD)
(DWMWA_USE_IMMERSIVE_DARK_MODE): Define.
(w32_applytheme): New function.
(w32_createvscrollbar, w32_createhscrollbar, w32_createwindow):
Call 'w32_applytheme'.
(globals_of_w32fns): Load 'DwmSetWindowAttribute' and
'SetWindowTheme' from their DLLs, and initialize 'w32_darkmode'.
* src/w32.c (w32_get_resource): Accept an additional argument
instead of hard-coding REG_ROOT; callers changed.  (Bug#51404)

* etc/NEWS:
* doc/emacs/msdos.texi (Windows Misc): Document the new feature.