Silence macOS 10.13 deprecation notices (Bug#29643)

Silence macOS 10.13 deprecation notices (Bug#29643)

* src/nsfns.m (Fx_display_backing_store):
(Fx_display_save_under): Don't use NSBackingStoreRetained or
NSBackingStoreNonretained on macOS 10.13+.
* src/nsselect.m (symbol_to_nsstring):
(ns_string_to_symbol):
(nxatoms_of_nsselect): Replace NSGeneralPboard with
NSPasteboardNameGeneral.
* src/nsterm.h: #define NSPasteboardNameGeneral to NSGeneralPboard on
GNUstep and macOS < 10.13.
* src/nsterm.m (EmacsView::resetCursorRects):
(EmacsScroller::resetCursorRects): Don't use setOnMouseEntered on
macOS 10.13+.