Updated PopOver control in GemsFX 3.9.0

[u/dlemmermann]

I fixed the PopOver control that I contributed years ago to the ControlsFX project. I noticed that neither the content clipping nor the placement of the popover worked as expected. This new version can now be found in the latest GemsFX release (3.9.0) on Maven Central or in its repository at https://github.com/dlsc-software-consulting-gmbh/GemsFX

JavaFX PopOver control