Add a function to set monitor scale factor manually

Add a function to set monitor scale factor manually

* src/pgtkfns.c (pgtk_get_monitor_scale_factor): New function to get
manual scale factor from alist.
(Fpgtk_set_monitor_scale_factor): New function to set manual scale
factor into alist.
(Fx_display_pixel_width): Use manual scale factor first, if not set,
use gdk's one.
(Fx_display_pixel_height): Use manual scale factor first, if not set,
use gdk's one.
(Fpgtk_display_monitor_attributes_list): Use manual scale factor
first, if not set, use gdk's one.
(syms_of_pgtkfns): initialize alist.