Add customization option for using UTF-8 coordinates in xt-mouse

Add customization option for using UTF-8 coordinates in xt-mouse

* lisp/xt-mouse.el (xterm-mouse-utf-8): New customization option.
(xterm-mouse--read-coordinate): New function to replace
`xterm-mouse--read-utf8-char'; uses UTF-8 only if enabled.
(xterm-mouse--read-number-from-terminal): Adapt to new name.
(xterm-mouse-tracking-enable-sequence)
(xterm-mouse-tracking-disable-sequence): Replace constants with
functions, mark constants as obsolete.
(xterm-mouse--tracking-sequence): New helper function.
(turn-on-xterm-mouse-tracking-on-terminal): Use new functions;
enable UTF-8 only if customization option says so; store UTF-8
flag in terminal parameter.  (Bug#23009)

* test/automated/xt-mouse-tests.el: Add tests for xt-mouse.el.