Implement monitor change functions on GNUstep

Implement monitor change functions on GNUstep

* src/nsfns.m (Fns_display_monitor_attributes_list): Fix coding
style.
* src/nsterm.m (nstrace_leave, nstrace_restore_global_trace_state)
(nstrace_fullscreen_type_name): Fix coding style.
(ns_displays_reconfigured, ns_term_init): Make a record of the
previous display attributes list and avoid storing duplicate
events.
([EmacsApp init]): Listen for
NSApplicationDidChangeScreenParametersNotification.
([EmacsApp updateMonitors:]): New method.
(syms_of_nsterm): New staticpro.