UI: Warn when closing dock widgets for first time

Users don't realize that dockable windows can be closed (hidden) and can
be shown again via the View menu.  This adds an explicit warning when
the user first closes a dockable window for their first time.  In future
versions, this should be changed to a dialog box with a "Do not show
again" checkbox.

UI/window-dock.hpp

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <QDockWidget>
+
+class OBSDock : public QDockWidget {
+	Q_OBJECT
+
+public:
+	inline OBSDock(QWidget *parent = nullptr) : QDockWidget(parent) {}
+
+	virtual void closeEvent(QCloseEvent *event);
+};