Add more error checking to some X functions

Add more error checking to some X functions

* src/xfns.c (Fx_change_window_property, Fx_delete_window_property)
(Fx_window_property, Fx_window_property_attributes): Check for
errors in case the window is invalid.  Also use better function
for interning prop names.

* src/xterm.c (struct x_atom_ref): New structure.
(x_atom_refs): New list.  Extract from x_term_init.
(x_intern_cached_atom): New function.  Use it in several places
to avoid interning common atoms.
(x_term_init): Adjust accordingly.

* src/xterm.h: Update prototypes.