Move all layout definitions to Rust

We'd like to finish removing all definitions of layout content in Svelte, moving them instead to Rust. This continues the work recently set out in #3447, #3444, #3436, and #2380. This is a prerequisite for #900.

The following lists the locations of all frontend-defined content that will need to be moved to backend-defined layouts:

- ColorPicker.svelte (everything)
- Dialog.svelte (header section, footer section, and somehow the crash info)
- ~~NodeCatalog.svelte~~ (will be removed entirely)
- Layers.svelte (layer rows)
- Welcome.svelte (logo image, message of the day)
- ~~Graphite.svelte~~ (will be removed entirely)
- WidgetSection.svelte (section headers and hover buttons)
- WindowButtons*.svelte (all buttons)

This is a helpful search (with case sensitivity enabled):
```
<[A-Z]+(Input|Buttons?|Label)
```