UI: Don't use "quit on last window closed"

Instead, quit only when the main window has been fully closed (and not
minimized to tray).  Fixes a bug where if the main window is minimized
to tray and another non-child window is open (for example, the stats
window), and then that window is closed, would cause the program to
prematurely exit and crash.

UI/window-basic-main.cpp

@@ -2977,6 +2977,8 @@ void OBSBasic::closeEvent(QCloseEvent *event)
 	/* Clear all scene data (dialogs, widgets, widget sub-items, scenes,
 	 * sources, etc) so that all references are released before shutdown */
 	ClearSceneData();
+
+	App()->quit();
 }
 
 void OBSBasic::changeEvent(QEvent *event)