Allow watch faces to implement custom handleButtonPress functions

This PR allows users to implement custom implementations for handleButtonPress!

For example, with the changes in this PR, I could do:

void WatchyKeen::handleButtonPress() {
  pinMode(VIB_MOTOR_PIN, OUTPUT);

  digitalWrite(VIB_MOTOR_PIN, HIGH);
  delay(50);
  digitalWrite(VIB_MOTOR_PIN, LOW);

  Watchy::handleButtonPress();
}

This example gives a "tactile" effect for every button press by doing two things:

Vibrates the motor for 50 ms
Calls the original handleButtonPress from the Watchy library

These changes are backward-compatible: if a watch face does not implement handleButtonPress, then there will be no functional changes with any watch face.

In addition, calling Watchy::handleButtonPress() from the user's own handleButtonPress function will make the watch perform its regular functions with the original buttons. However, if the user would like to have their own menu, for example, they don't need to always call Watchy::handleButtonPress(). This could look like this:

bool inMyMenu = false;

void WatchyKeen::handleButtonPress() {
  if (!inMyMenu && buttonPressed == myMenuButton) {
    inMyMenu = true;
  }

  if (inMyMenu) {
    // perform your own tasks here
    // set inMyMenu to false when you're done
  } else {
    Watchy::handleButtonPress();
  }
}

Other examples of how this could be implemented:

Make the up and down buttons turn lights on and off in your home
Kick off a non-deep-sleep routine, like a watch face-specific menu or other feature
Change the watch backgrounds, if multiple are available on a watch face

sqfmi / Watchy

Allow watch faces to implement custom handleButtonPress functions #165