add loading indicator

src/plain-text/ActIndicator.ts

@@ -0,0 +1,11 @@
+export interface ActIndicatorText {
+  loading: string;
+}
+
+export const actIndicatorZhCn: ActIndicatorText = {
+  loading: '加载中...',
+};
+
+export const actIndicatorEnUs: ActIndicatorText = {
+  loading: 'Loading...',
+};