feat: props spread

[atty303]

(I'll write more details later.)

fixes #1179

[tigerros]

This would be really useful for #1483

[ealmloff]

This should be working now with a few caveats:

• The builder macro is getting very large. It works, but it is difficult to change. We should pull it out into multiple files. Because this PR unblocks a UI toolkit, I don't think this should be changed in this PR. I can refactor the macro in a new PR
• This PR does not handle overlapping attributes between elements you extend (eg. input and global attribute both have a width attribute). We need some form of specialization here. We also need an owned version to the builder. Deref specialization only gives you an immutable reference to the object. I remember some marker trait based specialization, but I can't find the article now. Any ideas here would be appreciated!

Matthunz pointed out https://github.com/search?q=repo%3Alinebender%2Fxilem+viewmarker&type=code might be useful here?

[atty303]

@ealmloff
I don't have enough time to contribute now, so I'll leave the rest to you. Thanks and sorry.

[jkelleyrtp]

@atty303 Thanks for the PR! Excited to get this out in 0.5!