Make sorting case insensitive

lib/core/sorting_mode.dart

@@ -187,13 +187,17 @@ int _sortTitleAsc(Note a, Note b) {
     return -1;
   }
   if (!aTitleExists && !bTitleExists) {
-    return a.fileName.compareTo(b.fileName);
+    return _sortFileNameAsc(a, b);
   }
-  return a.title.compareTo(b.title);
+  var aTitle = a.title.toLowerCase();
+  var bTitle = b.title.toLowerCase();
+  return aTitle.compareTo(bTitle);
 }
 
 int _sortFileNameAsc(Note a, Note b) {
-  return a.fileName.compareTo(b.fileName);
+  var aFileName = a.fileName.toLowerCase();
+  var bFileName = b.fileName.toLowerCase();
+  return aFileName.compareTo(bFileName);
 }
 
 NoteSortingFunction _reverse(NoteSortingFunction func) {

test/sorting_mode_test.dart

@@ -56,5 +56,30 @@ void main() {
       expect(notes[2], n3);
       expect(notes[3], n4);
     });
+
+    test('Title', () async {
+      var folder = NotesFolderFS(null, '/tmp/', Settings(''));
+      var n1 = Note(folder, '/tmp/1.md');
+      n1.title = "alpha";
+
+      var n2 = Note(folder, '/tmp/2.md');
+      n2.title = "beta";
+
+      var n3 = Note(folder, '/tmp/3.md');
+      n3.title = "Axios";
+
+      var n4 = Note(folder, '/tmp/4.md');
+      n4.title = "";
+
+      var notes = [n1, n2, n3, n4];
+      var sortFn = SortingMode(SortingField.Title, SortingOrder.Ascending)
+          .sortingFunction();
+
+      notes.sort(sortFn);
+      expect(notes[0], n1);
+      expect(notes[1], n3);
+      expect(notes[2], n2);
+      expect(notes[3], n4);
+    });
   });
 }