* lisp/term/xterm.el: Add gui-get-selection support via OSC-52

* lisp/term/xterm.el: Add gui-get-selection support via OSC-52

(xterm--extra-capabilities-type): Add `getSelection'.
(xterm--query): Add `no-async' argument.
(xterm--init-activate-get-selection): New function.
(terminal-init-xterm): Use it.
(xterm--init-modify-other-keys): Rename from
terminal-init-xterm-modify-other-keys.
(xterm--init-bracketed-paste-mode): Rename from
terminal-init-xterm-bracketed-paste-mode.
(xterm--init-activate-set-selection): Rename from
terminal-init-xterm-activate-set-selection.
(xterm--selection-char): New function.
(gui-backend-set-selection): Use it.  Use the &context to only apply
this method in terminals where we enabled the feature.
(gui-backend-get-selection): New method.