Allow Folders to be deleted

For safety we're only allowing non-empty folders to be deleted. Ideally,
we should have some kind of undo button. Or some kind of history - since
we do have that with git.

Related to #18

lib/core/git_repo.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:io';
 
 import 'package:fimber/fimber.dart';
 import 'package:flutter/foundation.dart';
@@ -86,6 +87,20 @@ class GitNoteRepository {
     return NoteRepoResult(noteFilePath: noteFilePath, error: false);
   }
 
+  Future<NoteRepoResult> removeFolder(String folderPath) async {
+    var gitDir = p.join(baseDirectory, dirName);
+    var pathSpec = folderPath.replaceFirst(gitDir, "").substring(1);
+
+    await _gitRepo.rm(pathSpec);
+    await _gitRepo.commit(
+      message: "Removed Folder " + pathSpec,
+    );
+
+    await Directory(folderPath).delete(recursive: true);
+
+    return NoteRepoResult(noteFilePath: folderPath, error: false);
+  }
+
   Future<NoteRepoResult> resetLastCommit() async {
     await _gitRepo.resetLast();
     return NoteRepoResult(error: false);

lib/core/notes_folder.dart

@@ -48,6 +48,23 @@ class NotesFolder with ChangeNotifier {
     return _entities.firstWhere((e) => e.isNote, orElse: () => null) != null;
   }
 
+  bool get hasNotesRecursive {
+    bool has = hasNotes;
+    if (has) return true;
+
+    for (var i = 0; i < _entities.length; i++) {
+      var e = _entities[i];
+      if (e.isNote) continue;
+
+      has = has || e.folder.hasNotes;
+      if (has) {
+        return true;
+      }
+    }
+
+    return has;
+  }
+
   int get numberOfNotes {
     int i = 0;
     _entities.forEach((e) {
@@ -231,6 +248,21 @@ class NotesFolder with ChangeNotifier {
     notifyListeners();
   }
 
+  void removeFolder(NotesFolder folder) {
+    folder.removeListener(_entityChanged);
+
+    var i = _entities.indexWhere((e) {
+      if (e.isNote) return false;
+      return e.folder.folderPath == folder.folderPath;
+    });
+    assert(i != -1);
+
+    _entities.removeAt(i);
+    _entityMap.remove(folder.folderPath);
+
+    notifyListeners();
+  }
+
   void rename(String newName) {
     var dir = Directory(folderPath);
     var parentDirName = dirname(folderPath);