Fixes for clippy, added documentation effort

[fschutt]

This branch adds missing information, as well as clippy lints. This fixes roughly 200 clippy lints and prevents lots of bugs, ranging from floating-point errors to performance issues (lots of types could implement Copy and Default, but didn't).

So that's for the clippy lints. But please, DOCUMENT stuff. I can't guess what functions are supposed to do in a bigger picture, which is horrible for maintenance. One comment per function at least, please.

Notable changes:

• Event is now Copy. Meaning, you don't have to call .clone() on it anymore.
• IdRef.next() has been renamed to IdRef.next_id() because it could be confused with std::iterator::Iterator::next().
• WidgetBuilder has a new function: remove_prop(), which removes a property
• WidgetBuilder can now accept both String and &str for naming

[christolliday]

Thanks for this, looks mostly good aside from the comments above, yeah documentation is a high priority for me now that people are actually looking at this project. Currently working on a documentation branch I'll merge sometime after this.