Display WiFi RSSI level on status bar

[brentru]

Add a timer callback to check the RSSI from the network adapter and display a wifi icon on the status bar.

[T-Mosher]

Bumping this, it would be really nice to know the RSSI for Wippersnapper WiFi connections. Maybe make it a Component that can be used as a feed.

[tyeth]

Linking adafruit/Wippersnapper_Components#126

And https://github.com/tyeth/Adafruit_Wippersnapper_Arduino/tree/add-rssi

[tyeth]

Just to update that this RSSI displayed in the status bar is being considered as part of the UI redesign, but won't be any time soon.
For now there is a change to print the RSSI value on each ping (MQTT keep alive every 5seconds) to the serial port as part of normal / debug logging.

[tyeth]

Further note that RSSI is printed in serial monitor, both on connect and during MQTT pings. It is also displayed for each network from a wifi scan if desired SSID is not found. (Also supported with multi-wifi networks as part of #376)