Add support for Improv-WiFi

[melkati]

Add support for Improv-WiFi to be able to set WiFi credentials from browser just after installing or later at any time.

It has some difficulties as the serial port is shared with the menu system and sensors... Will have to be sorted out.

• [x] Initial basic implementation
• [x] Test with TFT screens
• [ ] Test with OLED screens (different menu logic)
• [ ] Test without screen (different menu logic)
• [ ] Test compatibility with serial sensors
• [ ] Document for users to use

Things to do and fix out before making into release:

• [ ] Make display logic work as in development version (sometimes show garbage and flickers)

[melkati]

Improv-WiFi is already included in the development branch but sometimes you have to try more than once to "activate" improv (just move back and forth between the main menu and log in the web installer).

I don't know exactly where is problem is hiding, I will try again...

BTW it was a somewhat difficult integration as both Improv-WiFi and the menu system are "fighting" for the data in the serial port.