Fix bottom gap calculations for embedded controllers

[Kaspik]

• the bottom gap had incorrect calculation for iPads when controller is not starting at y: 0. This should fix it.
• Closes: A layout issue on the ipad in the side window mode. #141

[Janneman84]

Unfortunately this fix actually broke the positioning. I did some testing and I believe I found a way that works well in all cases. At least it works well for all examples in the MessageKit example app when presented as pagesheet on iPad, as well as side view mode.

For bottomGap you changed:
return window.frame.height - superView.convert(superView.frame, to: window).maxY

to:
return window.frame.height - window.convert(superView.frame, to: window).maxY

but try this instead:
return window.frame.height - superView.convert(superView.bounds, to: window).maxY

So basically change frame with bounds.

Can you give this a try and confirm this works for your use case as well?