Rewrite x_set_mouse_color to sync less.

Rewrite x_set_mouse_color to sync less.

We can track serial numbers of X requests and correlate error events
with the associated requests.  This way we can identify errors for
specific calls without having to use XSync after every one.

* src/xfns.c (enum mouse_cursor): New type.
(struct mouse_cursor_types, struct mouse_cursor_data): New types.
(mouse_cursor_types): New array listing the Lisp variables and default
cursor appearances for each cursor type.
(x_set_mouse_color_handler): New function; checks error event serial
number against submitted requests.
(x_set_mouse_color): Updated to use the new error handler callback,
and to be more table-driven, to simplify repetitious code.