Fix for issue #21

* Added buttons to toggle arrow visibility and arrow pointing direction
* Added property 'borderColor' to test 'visual artifacts' at arrowSize=0
* Added property 'arrowHeightFactor' to change arrow's width/height-ratio, which could be used to let the arrow point outside instead of inside if this factor has a minus-sign
* Change customizeTheme to be uglier (just for testing)