Do not reparse every note on any change

Earlier whenever anything changed we would discard all the Notes +
NotesFolders in memory, and reconstruct the hierarchy. This would means
Notes which haven't changed at all, still need to parsed all over again.

Now we only reload what has actually changed.

This should result in a huge performance boost.

We still shouldn't be loading everything on boot, but that's a different
problem.

lib/core/notes_folder.dart

@@ -8,10 +8,12 @@ import 'note.dart';
 import 'note_fs_entity.dart';
 
 class NotesFolder with ChangeNotifier {
-  NotesFolder parent;
+  final NotesFolder parent;
-  List<NoteFSEntity> _entities = [];
   String folderPath;
 
+  List<NoteFSEntity> _entities = [];
+  Map<String, NoteFSEntity> _entityMap = {};
+
   NotesFolder(this.parent, this.folderPath);
 
   @override
@@ -91,18 +93,25 @@ class NotesFolder with ChangeNotifier {
 
   // FIXME: This should not reconstruct the Notes or NotesFolders once constructed.
   Future<void> load() async {
-    final dir = Directory(folderPath);
+    Set<String> pathsFound = {};
-    _entities.forEach((e) {
-      if (e.isFolder) {
-        e.folder.dispose();
-      } else {
-        e.note.dispose();
-      }
-    });
-    _entities = [];
 
+    var entitiesAdded = false;
+    var entitiesRemoved = false;
+
+    final dir = Directory(folderPath);
     var lister = dir.list(recursive: false, followLinks: false);
     await for (var fsEntity in lister) {
+      if (fsEntity is Link) {
+        continue;
+      }
+
+      // If already seen before
+      var existingNoteFSEntity = _entityMap[fsEntity.path];
+      if (existingNoteFSEntity != null) {
+        pathsFound.add(fsEntity.path);
+        continue;
+      }
+
       if (fsEntity is Directory) {
         var subFolder = NotesFolder(this, fsEntity.path);
         if (subFolder.name.startsWith('.')) {
@@ -112,6 +121,10 @@ class NotesFolder with ChangeNotifier {
 
         var noteFSEntity = NoteFSEntity(folder: subFolder);
         _entities.add(noteFSEntity);
+        _entityMap[fsEntity.path] = noteFSEntity;
+
+        pathsFound.add(fsEntity.path);
+        entitiesAdded = true;
         continue;
       }
 
@@ -123,9 +136,32 @@ class NotesFolder with ChangeNotifier {
 
       var noteFSEntity = NoteFSEntity(note: note);
       _entities.add(noteFSEntity);
+      _entityMap[fsEntity.path] = noteFSEntity;
+
+      pathsFound.add(fsEntity.path);
+      entitiesAdded = true;
     }
 
-    if (_entities.isNotEmpty) {
+    Set<String> pathsRemoved = _entityMap.keys.toSet().difference(pathsFound);
+    pathsRemoved.forEach((path) {
+      var e = _entityMap[path];
+      assert(e != null);
+
+      if (e.isFolder) {
+        e.folder.removeListener(_entityChanged);
+      } else {
+        e.note.removeListener(_entityChanged);
+      }
+
+      _entityMap.remove(path);
+    });
+    _entities.removeWhere((e) {
+      String path = e.isFolder ? e.folder.folderPath : e.note.filePath;
+      return pathsRemoved.contains(path);
+    });
+
+    entitiesRemoved = pathsRemoved.isNotEmpty;
+    if (entitiesAdded || entitiesRemoved) {
       notifyListeners();
     }
   }