Move gradient picking into the color picker (#1778)

* Gradient picker

* Fix up color picker layout CSS problems

* Begin hooking up SpectrumInput for gradient in the ColorPicker

* Working gradient picking on the frontend only

* Plumb FillColorChoice into the backend

* Hook everything else up, just with a weird bug remaining

* Fix some svelty reactivity issues

* Add and remove stops

* Cleanup

* Rename type

* Fill node document format upgrading

* Fix lint

* Polish the color picker UX and fix a bug

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Co-authored-by: 0hypercube <0hypercube@gmail.com>

frontend/src/components/floating-menus/MenuList.svelte

@@ -523,7 +523,7 @@
 
 				.entry-icon,
 				.no-icon {
-					margin-left: 4px;
+					margin: 0 4px;
 				}
 
 				.user-input-label {