Add setting to adjust WiFi TX Power

[jgstroud]

Similar to the 802.11B/G/N setting, we should also allow adjusting the transmit power. Don't know what the default is, but the internet seems to says it defaults to max.

/**

• set the output power of WiFi
• @param dBm max: +20.5dBm min: 0dBm
  */
  void ESP8266WiFiGenericClass::setOutputPower(float dBm) {

WiFi.setOutputPower(0); // this sets wifi to lowest power
WiFi.setOutputPower(20.5); // this sets wifi to highest power

see also
https://gitlab.com/painlessMesh/painlessMesh/-/issues/51

[dkerr64]

Queued up in PR #145

Implemented as a range slider in the GUI with possible values from 0 to 20. If 20 is selected then when setting power it is "rounded up" to 20.5.

[jgstroud]

Fixed in #145