Add ability to change macOS WM theme (bug#27973)

Add ability to change macOS WM theme (bug#27973)

* src/frame.c (make_frame, frame_parms, syms_of_frame)
[NS_IMPL_COCOA]: Add ns-appearance and ns-transparent-titlebar
options.
* src/frame.h (ns_appearance_type) [NS_IMPL_COCOA]: Add enum to
represent NSAppearance options.
(struct frame) [NS_IMPL_COCOA]: Add ns_appearance and
ns_transparent_titlebar frame parameters.
* src/nsfns.m (ns_frame_parm_handlers) [NS_IMPL_COCOA]: Add
ns_set_appearance and ns_set_transparent_titlebar handlers.
(Sx_create_frame): Handle ns-appearance and ns-transparent-titlebar
frame parameters.
(Qdark): Add new symbol for use with ns-appearance.
* src/nsterm.h (ns_set_appearance, ns_set_transparent_titlebar)
[NS_IMPL_COCOA]: Add prototypes.
* src/nsterm.m (ns_set_appearance, ns_set_transparent_titlebar)
[NS_IMPL_COCOA]: New functions.
(initFrameFromEmacs) [NS_IMPL_COCOA]: Handle ns-appearance and
ns-transparent-titlebar frame parameters.
* doc/lispref/frames.texi (Window Management Parameters): Document
ns-apperance and ns-transparent-titlebar.