Move all layout definitions to Rust

[Keavon]

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)