UI: Do not show [x] (close) for primary dock widgets

This solves the issue of users who close the primary dock widgets and
can't figure out how to get them back, who for whatever reason choose
not to read the message box when closing dock widgets that tells them
how to get it back.  The users can still hide the primary dock widgets
via the View menu, but they can no longer close them via the [x] button
on the dock widget.  This change does not apply to secondary dock
widgets, such as a chat window, stats, etc.

Closes obsproject/obs-studio#1804

UI/window-basic-main.cpp

@@ -6503,11 +6503,14 @@ void OBSBasic::on_lockUI_toggled(bool lock)
 		? QDockWidget::NoDockWidgetFeatures
 		: QDockWidget::AllDockWidgetFeatures;
 
-	ui->scenesDock->setFeatures(features);
-	ui->sourcesDock->setFeatures(features);
-	ui->mixerDock->setFeatures(features);
-	ui->transitionsDock->setFeatures(features);
-	ui->controlsDock->setFeatures(features);
+	QDockWidget::DockWidgetFeatures mainFeatures = features;
+	mainFeatures &= ~QDockWidget::QDockWidget::DockWidgetClosable;
+
+	ui->scenesDock->setFeatures(mainFeatures);
+	ui->sourcesDock->setFeatures(mainFeatures);
+	ui->mixerDock->setFeatures(mainFeatures);
+	ui->transitionsDock->setFeatures(mainFeatures);
+	ui->controlsDock->setFeatures(mainFeatures);
 	statsDock->setFeatures(features);
 
 	for (int i = extraDocks.size() - 1; i >= 0 ; i--) {