MainWindow: Set objectName of dockWidget(s)

When we want to call QMainWindow::saveState(), we have to ensure the following:

"The objectName property is used to identify each QToolBar and QDockWidget. You should make sure that this property is unique for each QToolBar and QDockWidget you add to the QMainWindow"

Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>