Reduce number of unneeded copies

This patch aims at reducing the number of copies for obejcts that could
be referenced rather than copied, because they're not modified during
the computation.

src/gui/MainWindow.cpp

@@ -673,7 +673,7 @@ void MainWindow::switchToOpenDatabase()
     switchToDatabases();
 }
 
-void MainWindow::switchToDatabaseFile(QString file)
+void MainWindow::switchToDatabaseFile(const QString& file)
 {
     m_ui->tabWidget->openDatabase(file);
     switchToDatabases();