Subject: bridge_builtin_features

Add optional touch variable : TOUCH_MIXMONITOR_BEEP(interval)

Setting TOUCH_MIXMONITOR_BEEP/TOUCH_MONITOR_BEEP to a valid
interval in seconds will result in a periodic beep being
played to the monitored channel upon MixMontior/Monitor
feature start.

If an interval less than 5 seconds is specified, the interval
will default to 5 seconds.  If the value is set to an invalid
interval, the default of 15 seconds will be used.
