Fix missing welcome panel after closing all documents
Closing all documents left the Welcome panel as the only tab, but the old active document index could still point to a tab that no longer existed. The fix is to reset it to 0 when the document list becomes empty.
* Break out VisualColorPickersInput.svelte
* Break out ColorComparisonInput.svelte and ColorPresetsInput.svelte
* Add backend definitions and plumbing for the 4 new widgets
* Port the ColorPicker.svelte layout and business logic to Rust
* Port more ColorComparisonInput.svelte logic to Rust
* Port more SpectrumInput.svelte logic to Rust
* Port more frontend logic to Rust
* Code review
* Code review
* Fix some CSS
* Add interactive panel docking
* Preserve active tab when a panel group is docked
* Add inter-panel gutter hover color
* Code review fixes
* More code review
* Add support for dragging panel tabs docked into other panel tab bars
* Fix terminology
* Add Group suffix to PanelGroupId enums variants
* Code review
* Remove the unused Editor.raw/wasmMemory/wasmImport
* Split out Editor.subscriptions
* Replace editor.handle.* with editor.* (1 of 2)
* Replace editor.handle.* with editor.* (2 of 2)
* Replace Editor typedef with EditorHandle import
* Pluralize subscription-router and rename subscriptionsRef->subscriptionsRouter and editorRef->editorHandle
* Remove editor.ts
* Update the readme
* Fix demo art loading bug
* Replace parameter passing with getContext and extract destroy functions to module-level exports
* Resend layouts from Rust when editor is re-mounted on HMR
* Code review
* Clean up component setup/tear-down side effects
* Clean up more component setup/tear-down side effects
* Remove nonfunctional debouncer
* Clean up even more component setup/tear-down side effects
* Reuse backend state
* Fix HMR for IO Managers and for State Providers
* Rename IO Managers -> Managers and State Providers -> Stores
* Restructure and partially flatten managers/stores
* Code review fixes
* Review fixes
