[Spec] Extract INPC from Layout system

[PureWeen]

Make StackLayout work for the Handlers project

• Extract the layout logic of StackLayout to StackLayoutArranger and put StackLayoutArranger into the Xamarin.Platform.Handlers class library. StackLayoutArranger will ingest an IStackLayout and arrange the IViews that are part of the IStackLayout

• Existing layout system isn't set in stone and the format of certain methods should be validated for example Handlers xamarin/Xamarin.Forms#12102 (comment)

• [Spec] drop AndExpand layout option #29

• Change the behavior of Height and Width so that they are more deterministic. What does this mean for breaking changes? Can we break this stuff?

  • Minimum (H/W) when this is set the element will never go below this value
  • Maximum (H/W) when this is set the element will never go over this value
  • Users need the ability to specify and exact Height/Width. If you specify a Width or Height larger than the container the element will just overflow from the container
  • [Enhancement] Fix XF absolute layout design #169
  • [Enhancement] Make all default spacings / margins zero #39