Allow button press overrides

[khenderick]

This PR allows watch faces to override the button press handler. This means that a face can give different functionality to certain buttons, or leave others unchanged.

Add to .h:

class Face : public Watchy{
    using Watchy::Watchy;
    public:
        void drawWatchFace();
        // ...
        virtual void handleButtonPress();
};

void Face::handleButtonPress() {
  if (guiState == WATCHFACE_STATE) {
    uint64_t wakeupBit = esp_sleep_get_ext1_wakeup_status();
    if (wakeupBit & UP_BTN_MASK) {
      // Do something special
      RTC.read(currentTime);
      showWatchFace(false);
      return;
    }
    if (wakeupBit & DOWN_BTN_MASK) {
      // Do something else special
      RTC.read(currentTime);
      showWatchFace(false);
      return;
    }
    if (wakeupBit & BACK_BTN_MASK) {
      // if (something special) {
      //   restore_to_normal();
      //   return;
      // }
      Watchy::handleButtonPress();
      return;
    }
    if (wakeupBit & MENU_BTN_MASK) {
      Watchy::handleButtonPress();
      return;
    }
  } else {Watchy::handleButtonPress();}
  return;
}

[sqfmi]

Looks good, thanks!