UI: Do not quit program when last windows are projectors

Prevents the program from unintentionally quitting if the program is
minimized to the task tray and the user closes all projectors

Closes jp9000/obs-studio#737

UI/window-projector.cpp

@@ -19,6 +19,9 @@ OBSProjector::OBSProjector(QWidget *widget, obs_source_t *source_)
 {
 	setAttribute(Qt::WA_DeleteOnClose, true);
 
+	//disable application quit when last window closed
+	setAttribute(Qt::WA_QuitOnClose, false);
+
 	installEventFilter(CreateShortcutFilter());
 
 	auto addDrawCallback = [this] ()