Add vertical slider orientation support.

[rterheide]

Added support for using egui::SliderOrientation to create vertical sliders.

[hacknus]

wow, nice, thanks @rterheide !
I will look into it in detail later, but for now it would be great to have an example (can be added to the existing example.rs or as a standalone one I think) and the changelog to be updated.

[rterheide]

Hi, thank you for your work with this project. Its been really great to work with

I added to the changelog and the example as you requested.

[hacknus]

this looks good to me, thanks a lot for your contribution!