Load previous state in alt text modal (#546)

src/lib/media/alt-text.ts

@@ -1,9 +1,13 @@
 import {RootStoreModel} from 'state/index'
 
-export async function openAltTextModal(store: RootStoreModel): Promise<string> {
+export async function openAltTextModal(
+  store: RootStoreModel,
+  prevAltText: string,
+): Promise<string> {
   return new Promise((resolve, reject) => {
     store.shell.openModal({
       name: 'alt-text-image',
+      prevAltText,
       onAltTextSet: (altText?: string) => {
         if (altText) {
           resolve(altText)