Border or frame support

Hey,
I came across the need to add a border to a Box element on my GUI application and found out that toga doesn't support borders yet. At first, I thought it is a problem only in Windows since many features haven't been implemented there yet, but now I see it is relevant for all platforms.

Is it on purpose? Or maybe you just haven't got the time to implement it yet?

I need 4 abilities that would really help me:

1. Border existence (hide/display)
2. Border size
3. Border color
4. Border style (dashed, dotted, etc.)

I work with python 3.7 on windows, on the latest toga version.

Start at the Box element and trace how its appearance is implemented on each supported platform. Confirm the API and platform behavior needed for border visibility, size, color, and styles, with completion requiring those four abilities to work consistently across platforms.

Written by the indexing model from the issue text.