feat: customise export dialog with UIOptions.canvasActions.export prop (#3658)

* refactor: update UIOptions.canvasActions.export to be a an object

* fix

* fix

* dnt show export icon when false

* fix

* inline

* memoize UIOptions

* update docs

* fix

* tweak readme

Co-authored-by: David Luzar <luzar.david@gmail.com>

src/excalidraw-app/index.tsx

@@ -424,7 +424,11 @@ const ExcalidrawWrapper = () => {
         onCollabButtonClick={collabAPI?.onCollabButtonClick}
         isCollaborating={collabAPI?.isCollaborating()}
         onPointerUpdate={collabAPI?.onPointerUpdate}
-        onExportToBackend={onExportToBackend}
+        UIOptions={{
+          canvasActions: {
+            export: { onExportToBackend },
+          },
+        }}
         renderTopRightUI={renderTopRightUI}
         renderFooter={renderFooter}
         langCode={langCode}