added isAppearing to prevent double render

[jensvrai]

The only real change is the adding of isAppearing function. Still needed 6 commits though :)

  /// Checks if the flushbar is appearing
  bool isAppearing() {
    return _flushbarRoute?.currentStatus == FlushbarStatus.IS_APPEARING;
  }

In my project I'm loading a flushbar when certain conditions are met, but sometimes I got double renders. But when I check the isAppearing()

Flushbar offlineFlushbar = Flushbar<bool>(
  message: "Je bent offline",
);

In my builder

WidgetsBinding.instance.addPostFrameCallback((_) {
  if (!offlineFlushbar.isShowing() &&
      !offlineFlushbar.isAppearing()) {
    offlineFlushbar.show(context);
  }
});

[JesseScott]

@jensvrai you're going to want to revert this change
@AndreHaueisen this probably should have been gitignored?

[danielRi]

Any news on this? We need this fix.