UI: Store dock titles in BrowserDock instead of relying on Qt

Attempting to set the window title of a BrowserDock that is closed, then
opening the BrowserDock, would show that BrowserDock with an incorrect
title. We can handle this by overriding the showEvent of BrowserDock and
manually setting the window title after the showEvent is called,
hopefully ensuring that we are only setting the window title on a window
that exists.

UI/window-dock.cpp

@@ -34,3 +34,8 @@ void OBSDock::closeEvent(QCloseEvent *event)
 
 	QDockWidget::closeEvent(event);
 }
+
+void OBSDock::showEvent(QShowEvent *event)
+{
+	QDockWidget::showEvent(event);
+}