Save splitter sizes of the Execute SQL areas to disk under request

Saving cannot be done in the class destructor, since it seems that the
sizes have already change in that moment. Now they are saved under request
of the Main Window at the same time when the Main Window state is saved.

See issue #1889.

src/MainWindow.cpp

@@ -967,6 +967,8 @@ void MainWindow::closeEvent( QCloseEvent* event )
         Settings::setValue("SchemaDock", "dropQualifiedNames", ui->actionDropQualifiedCheck->isChecked());
         Settings::setValue("SchemaDock", "dropEnquotedNames", ui->actionEnquoteNamesCheck->isChecked());
 
+        SqlExecutionArea::saveState();
+
         QMainWindow::closeEvent(event);
     } else {
         event->ignore();