Split widget callbacks into update and commit so only the latter adds a history state (#1584)

* feat: split commit and update layout

* feat: add on_commit callback

* Code review

* fix: refactor

---------

Co-authored-by: Keavon Chambers <keavon@keavon.com>

frontend/src/components/widgets/buttons/ColorButton.svelte

@@ -7,7 +7,7 @@
 	import LayoutCol from "@graphite/components/layout/LayoutCol.svelte";
 	import TextLabel from "@graphite/components/widgets/labels/TextLabel.svelte";
 
-	const dispatch = createEventDispatcher<{ value: Color }>();
+	const dispatch = createEventDispatcher<{ value: Color; startHistoryTransaction: undefined }>();
 
 	let open = false;
 
@@ -31,6 +31,11 @@
 			value = detail;
 			dispatch("value", detail);
 		}}
+		on:startHistoryTransaction={() => {
+			// This event is sent to the backend so it knows to start a transaction for the history system. See discussion for some explanation:
+			// <https://github.com/GraphiteEditor/Graphite/pull/1584#discussion_r1477592483>
+			dispatch("startHistoryTransaction");
+		}}
 		{allowNone}
 	/>
 </LayoutCol>