NoteEditor: Only enable 'Discard Changes' option if we have changes

Otherwise disable the button.

lib/editors/checklist_editor.dart

@@ -279,6 +279,9 @@ class ChecklistEditorState extends State<ChecklistEditor>
       _noteModified = true;
     });
   }
+
+  @override
+  bool get noteModified => _noteModified;
 }
 
 typedef TextChangedFunction = void Function(String);

lib/editors/common.dart

@@ -26,6 +26,8 @@ abstract class Editor {
 abstract class EditorState {
   Note getNote();
   Future<void> addImage(File file);
+
+  bool get noteModified;
 }
 
 class EditorAppBar extends StatelessWidget implements PreferredSizeWidget {
@@ -214,6 +216,7 @@ Widget _buildBottomMenuSheet(
 
             editor.discardChangesSelected(note);
           },
+          enabled: editorState.noteModified,
         ),
         ListTile(
           leading: Icon(Icons.share),

lib/editors/journal_editor.dart

@@ -123,6 +123,9 @@ class JournalEditorState extends State<JournalEditor> implements EditorState {
       _noteModified = true;
     });
   }
+
+  @override
+  bool get noteModified => _noteModified;
 }
 
 class _NoteBodyEditor extends StatelessWidget {

lib/editors/markdown_editor.dart

@@ -163,6 +163,9 @@ class MarkdownEditorState extends State<MarkdownEditor> implements EditorState {
       _noteModified = true;
     });
   }
+
+  @override
+  bool get noteModified => _noteModified;
 }
 
 class _NoteBodyEditor extends StatelessWidget {

lib/editors/raw_editor.dart

@@ -119,6 +119,9 @@ class RawEditorState extends State<RawEditor> implements EditorState {
       _noteModified = true;
     });
   }
+
+  @override
+  bool get noteModified => _noteModified;
 }
 
 class _NoteEditor extends StatelessWidget {