Compatibility adjustments to ensure react-layer-stack works in React 15.5+ and a small enhancement

[Sigma-90]

Hi Alexey,

I've already provided an issue regarding this, but in the meantime I also fixed it myself in a fork, so I thought it might be helpful to provide it here.

What I did in this fork are two things:

• First and foremost, I fixed the PropTypes dependency issue that caused the module to crash in React 15.5+.
• I also added two new props to LayerStackMount point that allow to assign custom CSS classes to the generated wrapper elements defined by elementType.

I also experimented a bit to check if it would also be possible to assign additional classes to those wrappers when there are actively shown layers (think ".layer-stack-wrapper.has-active-layers > .layer-wrapper.is-active") but that didn't work as expected because the LayerStack does not seem to re-render itself when a change in the layer stack happens (only the LayerMountPoints change), so I threw that out again and ended up with this simplified version.

If you approve of those two additional props, I'd appreciate if you would accept this pull request so that your awesome module works once more in the current version of react.

Beste regards,
Stefan

[lexfrl]

That's amazing! Thank you!
I'll check it as soon as possible!