Fix macOS/GNUstep compilation warnings

Fix macOS/GNUstep compilation warnings

* src/nsfns.m (handlePanelKeys): Unused function.
* src/nsterm.m (ns_set_appearance):
([EmacsView initFrameFromEmacs:]): Use EmacsWindow type instead of
NSWindow.
(ns_clip_to_row): Unused function.
(ns_dumpglyphs_stretch): Remove unused variable.
(ns_term_init):
([EmacsWindow setAppearance]): Only compile on macOS.
(ns_mouse_position): Make sure f is initialised on GNUstep.
* src/emacs.c (main): Move allocation of autorelease pool to before
first use.