Use the rich text char format to store mentions rather than a separate structure in ChatBarCache.

This removes mentions from ChatBarCache and instead sets mentions as an anchor using the QTextCursor. Saving and restoring the chatbar text content is then done using QTextDocumentFragments which retain the rich text formatting.

autotests/modeltest.cpp

@@ -400,7 +400,6 @@ void ModelTest::testCompletionModel()
     auto model = new CompletionModel(this);
     auto tester = new QAbstractItemModelTester(model, model);
     tester->setUseFetchMore(true);
-    model->setRoom(room);
     model->setAutoCompletionType(CompletionModel::Room);
     auto roomListModel = new RoomListModel(this);
     roomListModel->setConnection(connection);