Shadow the window behind the modal when it's open

[savchenko]

Is your feature request related to a problem? Please describe.
Opened modals blend with the rest of the window.

Describe the solution you'd like
Tint the background when a modal is open. Exactly what NeoVim does:

Describe alternatives you've considered
Just deal with it.

Additional context
Screenshots above.

[extrawurst]

That’s a fantastic idea. Not gonna be easy but I like it 👍