Floem rewrite

[pieterdd]

I was bumping into various kinds of friction with Iced that Floem doesn't seem to have so I'm implementing a test with Floem. If successful, Prettygoeey will become a Floem UI library.

To do:

• Reimplement existing widgets in Floem
• Write new documentation
• Refer Iced users to the last Iced-compatible version
• Address that issue with button background color after clicking
• Address issues with disabled state
• Wait for Propagate disabled state to descendants lapce/floem#231 to be merged, then update Cargo.toml

The UI elements to reimplement:

• Primary container
• Header
• Button
• Text (including dimmed variation)
• Radio group
• Checkbox
• Text input
• Slider Numeric input

So far advantages seem to be:

• Simple styling API, advanced styling functionality
• Easier to compose widgets of widgets (e.g. the spinbox use case)
• The overall workflow seems more ergonomic

Major caveat: Floem doesn't have an officially published crate yet. That said, the Lapce IDE - produced by the same devs - already uses it (Git dependency). So I don't expect drastic API changes.