Expose flushbar status + add isVisible method

[rico237]

@AndreHaueisen
Now every developer can have direct access to the current status of the flushbar

Bonus:

Added a isVisible method, to know if flushbar is currently displayed on screen.
This avoid flushbar to be shown multiple times when user spam a button action while status is appearing or hiding.

Example code:

if (_flushbar.isVisible() == false) {
    _flushbar.show(context);
}