Add a UI to indicate a pane is focused.

[zadjii-msft]

Misc. ramblings from my notes:

• IDEA: have the pane call some method on it's child controls to have it indroduce some margin, as opposed to creating some manual seperator. When the control is focused, colorize the margin. When it's unfocused, remove that color. Each pane optionally tells a control should it have margin on top/bottom/left/right, and does it recursively? so

[A|B]
A should have a right margin, b should have a left.

[A|[B-C]]
A has right
B has left, bottom
C has left, top

[A|[[B-D]-C]]
A has right
B has left, bottom(with D), bottom(with C)
C has left, top(with B-D)
D has left, top(with B), bottom(with C)

Margin doesn't stack, just is there or isn't
Margin color should be either null, accent, or a color (Global, Control)
Margin should have configurable width (Global, Control)

How does this interact with #992? Can we detect a click is in the margin, and surface that up? or can the terminal app listen to the control's click/drag, and handle it somehow?

[mdtauk]

This could use the FocusVisual or add a shadow to elevate the pane?

https://docs.microsoft.com/en-us/windows/uwp/design/style/reveal-focus

[zadjii-msft]

So, I considered FocusVisual, but all that documentation seems to suggest that it should be used for apps for 10-foot experiences, such as Xbox. Considering we're not planning on shipping on xbox any time soon, I assumed this wasn't the right effect to use.

I was definitely considering some sort of drop-shadow effect on panes in general. This might be a good conversation for @cinnamon-msft to have with our UI person