Add three new session options: visual-activity, visual-bell, visual-content. If

these are enabled (and the monitor-activity, bell-actio and monitor-content
options are configurated appropriately), when activity, a bell, or content is
detected, a message is shown.

Also tidy up the bell/activity/content code in server.c slightly and fix a
couple of errors.

tmux.1

@@ -1324,6 +1324,30 @@ and
 .Ic status-right
 strings as UTF-8; notably, this is important for wide characters.
 This option defaults to off.
+.It Xo Ic visual-activity
+.Op Ic on | Ic off
+.Xc
+If on, display a status line message when activity occurs in a window
+for with the
+.Ic monitor-activity
+window option is enabled.
+.It Xo Ic visual-bell
+.Op Ic on | Ic off
+.Xc
+If this option is on, a message is shown on a bell instead of it being passed
+through to the terminal (which normally makes a sound).
+Also see the
+.Ic bell-action
+option.
+.It Xo Ic visual-content
+.Op Ic on | Ic off
+.Xc
+Like
+.Ic visual-activity ,
+display a message when content is present in a window
+for with the
+.Ic monitor-content
+window option is enabled.
 .El
 .It Xo Ic set-password
 .Op Fl c