Add detection for assigning Color/Brush resources correctly

[mrlacey]

Currently, when a color resource is assigned to a property that takes a brush it is silently ignored.
The same is true for assigning a brush to a color.

This can lead to things not being drawn as expected at runtime and they can be hard to spot/debug.

Reproduce by creating a color resource and assigning it to a Fill or a SolidColorBrush resource and assigning it to a color.

Will need knowledge of styles to be able to do this. (So possibly a post-1.0 task)

Verified with Xamarin.Forms on VS2019 but need to check to see what else is affected.