feat: Add option to draw shadow behind toasts

[woelper]

This PR exposes the option to use a shadow on notifications in order to improve visibility. The shadow is optional.

[bircni]

Maybe also extend the examples to show the shadow

[woelper]

Maybe also extend the examples to show the shadow

I added a button to add shadows. Hope this works for you!

[bircni]

Could not spot any difference
with shadow:

without shadow:

[bircni]

I suggest changing the examples like this:
all.rs.txt

[woelper]

I suggest changing the examples like this: all.rs.txt

Done!