Factor out common code for multimonitors, for Gtk+, non-Gtk+ X11 and NS.

Factor out common code for multimonitors, for Gtk+, non-Gtk+ X11 and NS.

* frame.c (Qgeometry, Qworkarea, Qmm_size, Qframes, Qsource): New
Lisp_Object:s.
(free_monitors, make_monitor_attribute_list): New functions.
(syms_of_frame): DEFSYM Qgeometry, Qworkarea, Qmm_size, Qframes,
Qsource.

* frame.h (Qgeometry, Qworkarea, Qmm_size, Qframes, Qsource): Declare.
(struct MonitorInfo): New struct.
(free_monitors, make_monitor_attribute_list): Declare.

* nsfns.m (Qgeometry, Qworkarea, Qmm_size, Qframes, Qsource): Remove.
(struct MonitorInfo, free_monitors): Remove.
(ns_screen_name): Make static.
(ns_make_monitor_attribute_list): Call make_monitor_attribute_list.
(syms_of_nsfns): Remove DEFSYM for Qgeometry, Qworkarea, Qmm_size,
Qframes, Qsource.

* xfns.c (Qgeometry, Qworkarea, Qmm_size, Qframes, Qsource): Remove.
(struct MonitorInfo, free_monitors): Remove.
(x_make_monitor_attribute_list): Call make_monitor_attribute_list.
(Fx_display_monitor_attributes_list): Call make_monitor_attribute_list.
(syms_of_xfns): Remove DEFSYM for Qgeometry, Qworkarea, Qmm_size,
Qframes, Qsource.