[WiFi] Cleanup code, split ESP32/ESP8266 specific

[TD-er]

Finally going to clean up the WiFi code and restructure it.
Hopefully this will also show where the long lasting WiFi reconnect issues may be...

Fixes: #3870

ToDo:

• Fix WiFi Captive portal
• Load CSS on /setup page when connected via softAP
• Keep softAP active as long as some client is connected (WiFi.softAPgetStationNum() does not work as expected on ESP8266)
• Allow to keep AP_only mode active
• Perform WiFi scan on /setup page when "Rescan" button is pressed
• Call WiFi_AP_Candidates.force_reload() when saving security settings
• Make clear state whether we're still in "setup" mode
• Re-open AP mode if in "setup" mode and login failed
• Cleanup old code
• Test hidden SSID functionality
• Test static IP functionality

[TD-er]

Notes/bookmarks:

AP API: https://github.com/espressif/arduino-esp32/blob/master/libraries/WiFi/src/WiFiAP.h#L44-L57
STA API: https://github.com/espressif/arduino-esp32/blob/master/libraries/WiFi/src/WiFiSTA.h#L49-L83
ETH API: https://github.com/espressif/arduino-esp32/blob/master/libraries/Ethernet/src/ETH.h#L125-L163
PPP API: https://github.com/espressif/arduino-esp32/blob/master/libraries/PPP/src/PPP.h#L26-L80
Common for every interface: https://github.com/espressif/arduino-esp32/blob/master/libraries/Network/src/NetworkInterface.h#L39-L76

Manager api:
Main: https://github.com/espressif/arduino-esp32/blob/master/libraries/Network/src/NetworkManager.h#L17-L31
Events: https://github.com/espressif/arduino-esp32/blob/master/libraries/Network/src/NetworkEvents.h#L143-L219

Network.setDefaultInterface(NetworkInterface &ifc); takes as argument STA, ETH or PPP
This allows to set which of those is used for Internet.