WiFi event handling refactoring

[igrr]

Related: #2026.
This commit changes the API for subscribing to WiFi events.

Currently there are two functions: WiFi.onEvent and WiFi.removeEvent. The first one allows one to listen to WiFi events but doesn't pass along any useful information about the event. Then other one has a bug and doesn't work in some cases.
Proposed changes are as follows:

• WiFi.onEvent is kept but marked deprecated. It should not be used in new code.

• WiFi.removeEvent is removed.

• A bunch of new functions are added:

  WiFiEventHandler onStationModeConnected(std::function<void(const WiFiEventStationModeConnected&)>);
  WiFiEventHandler onStationModeDisconnected(std::function<void(const WiFiEventStationModeDisconnected&)>);
  WiFiEventHandler onStationModeAuthModeChanged(std::function<void(const WiFiEventStationModeAuthModeChanged&)>);
  WiFiEventHandler onStationModeGotIP(std::function<void(const WiFiEventStationModeGotIP&)>);
  WiFiEventHandler onStationModeDHCPTimeout(std::function<void(void)>);
  WiFiEventHandler onSoftAPModeStationConnected(std::function<void(const WiFiEventSoftAPModeStationConnected&)>);
  WiFiEventHandler onSoftAPModeStationDisconnected(std::function<void(const WiFiEventSoftAPModeStationDisconnected&)>);

  These functions take a function object or a function pointer, and return a WiFiEventHandler instance. WiFiEventHandler is an opaque type which represents an active subscription to WiFi events. It helps manage un-subscription from WiFi events by binding it to object lifetime.

Consider the following example

#include <Schedule.h>
#include <ESP8266WiFi.h>
class FooServer {
public:
    FooServer()
    : mServer(5555)
    {
    }

    void begin()
    {
        mConnectHandler = WiFi.onStationModeGotIP(std::bind(&FooServer::onGotIP, this, std::placeholders::_1));
        mDisconnectHandler = WiFi.onStationModeDisconnected(std::bind(&FooServer::onDisconnected, this, std::placeholders::_1));      
    }

protected:
    void onGotIP(const WiFiEventStationModeGotIP& event) 
    {
        // the module is connected to WiFi and got IP from DHCP server
        // run startServer function on main loop
        schedule_function(std::bind(&FooServer::startServer, this));
    }

    void onDisconnected(const WiFiEventStationModeDisconnected& event)
    {
        Serial.println("Stopping server");
        mServer.stop();
    }

    void startServer()
    {
        Serial.println("Starting server");
        mServer.begin();
    }

protected:
    IPAddress mAddr;
    WiFiEventHandler mConnectHandler;
    WiFiEventHandler mDisconnectHandler;
    WiFiServer mServer;
};

FooServer fooServer;

void setup() {
  Serial.begin(115200);
  Serial.setDebugOutput(true);
  fooServer.begin();
}

void loop() {
}

If a FooServer instance goes out of scope or is deleted, mConnectHandler and mDisconnectHandler will be deleted as well, automatically cancelling subscription to GotIP and Disconnected events.

[codecov-io]

Current coverage is 42.59%

Merging #2119 into master will increase coverage by 16.29%

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+ Partials        165          0   -165   

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