Add virtual miniscreen (display and buttons) web interface

[m-roberts]

I am imagining this as a backend of the miniscreen class - it could be configured to output to display/web/imgcat/pygame, etc.
An alternative would be to provide it as a swap-out replacement for pitop.miniscreen.Miniscreen

• perhaps pitop.miniscreen.WebEmulator?

This is obviously a different paradigm to other web interfaces in the SDK, but the advantage of doing this separately for the miniscreen is that this would be great for rapid debugging, local development (no pi-top), etc.

[hramrach]

Ins't this the same as #485 ?

[m-roberts]

Closed #485 in favour of this one. Thanks @hramrach