Rewrite of tmux mouse support which was a mess. Instead of having

options for "mouse-this" and "mouse-that", mouse events may be bound as
keys and there is one option "mouse" that turns on mouse support
entirely (set -g mouse on).

See the new MOUSE SUPPORT section of the man page for description of the
key names and new flags (-t= to specify the pane or window under mouse
as a target, and send-keys -M to pass through a mouse event).

The default builtin bindings for the mouse are:

    bind -n   MouseDown1Pane select-pane -t=; send-keys -M
    bind -n MouseDown1Status select-window -t=
    bind -n   MouseDrag1Pane copy-mode -M
    bind -n MouseDrag1Border resize-pane -M

To get the effect of turning mode-mouse off, do:

    unbind -n MouseDrag1Pane
    unbind -temacs-copy MouseDrag1Pane

The old mouse options are now gone, set-option -q may be used to
suppress warnings if mixing configuration files.

status.c

@@ -118,9 +118,9 @@ status_redraw_get_right(struct client *c, time_t t, int utf8flag,
 	return (right);
 }
 
-/* Set window at window list position. */
-void
-status_set_window_at(struct client *c, u_int x)
+/* Get window at window list position. */
+struct window *
+status_get_window_at(struct client *c, u_int x)
 {
 	struct session	*s = c->session;
 	struct winlink	*wl;
@@ -130,12 +130,13 @@ status_set_window_at(struct client *c, u_int x)
 	x += c->wlmouse;
 	RB_FOREACH(wl, winlinks, &s->windows) {
 		oo = &wl->window->options;
-
 		len = strlen(options_get_string(oo, "window-status-separator"));
-		if (x < wl->status_width && session_select(s, wl->idx) == 0)
-			server_redraw_session(s);
+
+		if (x < wl->status_width)
+			return (wl->window);
 		x -= wl->status_width + len;
 	}
+	return (NULL);
 }
 
 /* Draw status for client on the last lines of given context. */