[Mac] Autoresizing implementation

[CartBlanche]

[CartBlanche]

@viniciusjarina & @chkn This is the port of the Autoresizing control into proppy. Could you please look over it and the video I posted on Slack in #proppy (https://xamarinhq.slack.com/archives/C5GBTFXL0/p1568909239055500). It Supports theming and keyboard navigation (Left and Right arrows to cycle through the flags). With theming we need to decide what the actual colour scheme will be between dark and light, but that should be easily changed once we finalise what that will actually be.

[ermau]

@CartBlanche Theming is here #616 (comment)

[ermau]

You'll want to rebase this and use the rectangle sizing methodology now present in the view model as it is more correct.

[CartBlanche]

@ermau just to be clear, you are referring to VM.GetPreviewElementRectangle?

[ermau]

@CartBlanche Correct.

[CartBlanche]

I made it a vertical alignment as it really is better aesthetically than the horizontal one that doesn't support resizing. We can always revisit the alignment, should Forms be needed.