Centralise check of CONF_no_mouse_rep into Terminal.

This removes code duplication between the front ends: now the terminal
itself knows when the Conf is asking it not to turn on mouse
reporting, and the front ends can assume that if the terminal asks
them to then they should just do it.

This also makes the behaviour on mid-session reconfiguration more
sensible, in both code organisation and consistent behaviour.
Previously, term_reconfig would detect that CONF_no_mouse_rep had been
*set* in mid-session, and turn off mouse reporting mode in response.
But it would do it by clearing term->xterm_mouse, which isn't how the
front end enabled and disabled that feature, so things could get into
different states from different sequences of events that should have
ended up in the same place.

Also, the terminal wouldn't re-enable mouse reporting if
CONF_no_mouse_rep was *cleared* and the currently running terminal app
had been asking for mouse reports all along. Also, it was silly to
have half the CONF_no_mouse_rep handling in term_reconfig and the
other half in the front ends.

Now it should all be sensible, and also all centralised.
term->xterm_mouse consistently tracks whether the terminal application
is _requesting_ mouse reports; term->xterm_mouse_forbidden tracks
whether the client user is vetoing them; every change to either one of
those settings triggers a call to term_update_raw_mouse_mode which
sets up the front end appropriately for the current combination.

unix/gtkwin.c

@@ -2403,7 +2403,6 @@ static void gtk_seat_set_busy_status(Seat *seat, BusyStatus status)
 static void gtkwin_set_raw_mouse_mode(TermWin *tw, bool activate)
 {
     GtkFrontend *inst = container_of(tw, GtkFrontend, termwin);
-    activate = activate && !conf_get_bool(inst->conf, CONF_no_mouse_rep);
     inst->send_raw_mouse = activate;
     update_mouseptr(inst);
 }