Take root widget directly in WindowDesc::new

[lassipulkkinen]

WindowDesc::new currently accepts the root widget as a closure that returns W: Widget<_>, which made sense before the changes in #519. This PR changes that to take W directly instead, as using a closure serves no purpose anymore.

EDIT: Sorry for the force push mess; I forgot to cargo fmt. Also, reverted AUTHORS change on second thought, as this is probably not eligible for copyright.

[cmyr]

/bloat

[cmyr]

ha funny, I'd forgotten why this API was like this, but I'm pretty sure you're right.

I do think that this is a simplification, and I'm happy to merge, once we figure out what's up with CI.

[github-actions]

🗜 Bloat check ⚖️

Comparing ef17edd against 4c865b9

target	old size	new size	difference
target/release/examples/calc	1.42 MB	1.42 MB	56 Bytes (0.00%)
target/release/examples/scroll_colors	1.41 MB	1.41 MB	320 Bytes (0.02%)
target/release/examples/multiwin	1.42 MB	1.42 MB	-16 Bytes (-0.00%)
target/release/examples/flex	1.85 MB	1.85 MB	-48 Bytes (-0.00%)
target/release/examples/styled_text	1.64 MB	1.64 MB	-56 Bytes (-0.00%)
target/release/examples/custom_widget	1.33 MB	1.33 MB	96 Bytes (0.01%)