xterm and OSC 52: Add NEWS entry, and tweak the code

xterm and OSC 52: Add NEWS entry, and tweak the code

* lisp/term/xterm.el (gui-set-selection) <nil>: Move method definition to
top-level.
(terminal-init-xterm-activate-set-selection): Set a terminal property.
(xterm--set-selection): Use it instead of checking the value of
`terminal-initted'.  Don't use string-bytes.