Refactor GroupBox elevation

[nicolaihenriksen]

As mentioned in discussion #3201, there is something that does not work as expected with the elevation in combination with GroupBox.

The demo app already includes a sample of a GroupBox with elevation only applied on the header (ColorZone), so I am assuming this is a needed feature, and thus this PR will retain it.

However, if the GroupBox has a border, or a background, or is using the "card" style, then I would also assume elevation should work, but that does not seem to be the case.

It currently looks like this:

You'll notice that the elevation is still only applied at the header. This seems wrong to me.

This PR changes it into this instead:

Since the drop shadow is done using an DropShadowEffect, there is a bit of funkyness to this. If the GroupBox.Background property is left at its default value (Transparent), then the effect is applied only at the header to retain the current feature. However, if the it is set to any other value, then the effect is applied at the root grid instead, which effectively - in the case of a "reasonably opaque" brush - will let the shadow cover the whole body of the GroupBox. You can of course provide a funky value with a very low opacity and then you may not get the result you want. I could not really see any other good way to solve this. Hints/input is very welcome.

Another thing: I removed the use of the ShadowAssist.ShadowEdges attached property as I don't believe it has any effect. Perhaps it needs to be removed completely as part of the 5.0 release?