add option to toggle self-encrypt for files

OpenKeychain/src/main/res/menu/encrypt_file_fragment.xml

@@ -35,6 +35,12 @@
         android:orderInCategory="1"
         android:checkable="true" />
 
+    <item
+        android:id="@+id/check_enable_self_encrypt"
+        android:title="@string/label_enable_self_encrypt"
+        android:orderInCategory="1"
+        android:checkable="true" />
+
     <item
         android:id="@+id/check_encrypt_filenames"
         android:title="@string/label_encrypt_filenames"