New function to request the status of the Access Point with a limit uptime setting

[Thommes17]

Changes:

• New private bool: _WifiAP_active => True if the AP is active
• New public function: WifiAP_active(int max_uptime_minutes) => returns True if the AP is active and False if not or if the maximum uptime in minutes has been reached

Background:
Limits the uptime of the AP for battery powered devices in case the Wifi is temporarily not accessible. In the code, the status of the AP can be requested and the device sen to sleep in case the maximum uptime has been exceeded instead of keeping the AP active.