Note Rename: Do not let it be renamed to a non markdown file

Otherwise it disappears from the listing as we only show files which end
with a '.md'.

lib/state_container.dart

@@ -151,6 +151,11 @@ class StateContainerState extends State<StateContainer> {
   }
 
   void renameNote(Note note, String newFileName) async {
+    // Do not let the user rename it to a non-markdown file
+    if (!newFileName.toLowerCase().endsWith('.md')) {
+      newFileName += '.md';
+    }
+
     var oldNotePath = note.filePath;
     note.rename(newFileName);