Prepare undo

app/src/main/res/drawable/baseline_undo_24.xml

@@ -0,0 +1,10 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+        android:width="24dp"
+        android:height="24dp"
+        android:viewportWidth="24.0"
+        android:viewportHeight="24.0"
+        android:tint="?attr/colorControlNormal">
+    <path
+        android:fillColor="@android:color/white"
+        android:pathData="M12.5,8c-2.65,0 -5.05,0.99 -6.9,2.6L2,7v9h9l-3.62,-3.62c1.39,-1.16 3.16,-1.88 5.12,-1.88 3.54,0 6.55,2.31 7.6,5.5l2.37,-0.78C21.08,11.03 17.15,8 12.5,8z"/>
+</vector>

app/src/main/res/menu/action_compose.xml

@@ -11,6 +11,12 @@
         android:icon="@drawable/baseline_save_alt_24"
         android:title="@string/title_save" />
 
+    <item
+        android:id="@+id/action_undo"
+        android:icon="@drawable/baseline_undo_24"
+        android:title="@string/title_undo"
+        android:visible="false" />
+
     <item
         android:id="@+id/action_send"
         android:icon="@drawable/baseline_send_24"