SqliteTableModel: Notify view and parent class code when clearing cache

Call beginRemoveRows() and endRemoveRows() when emptying the cache. This
silences those warnings from QAbstractItemModel::endInsertRows() and
fixes a crash when exiting the program both ocurring when the data was
resorted.

src/sqlitetablemodel.h

@@ -41,6 +41,7 @@ public slots:
 
 private:
     void fetchData(unsigned int from, unsigned to);
+    void clearCache();
 
     void buildQuery();