Qt4: fix crash regression in 1.1.5 closeAllWindows() not thread-safe

All UI related operations in Qt4 must be done on the UI thread, as
QtGui is not thread-safe internally.

modules/gui/qt4/qt4.cpp

@@ -392,14 +392,10 @@ static void Close( vlc_object_t *p_this )
     }
 
     /* And quit */
-    msg_Dbg( p_intf, "Please die, die, die..." );
-    QApplication::closeAllWindows();
-
-//    QApplication::quit();
-
+    msg_Dbg( p_this, "requesting exit..." );
     QVLCApp::triggerQuit();
 
-    msg_Dbg( p_intf, "Please die, die, die 2..." );
+    msg_Dbg( p_this, "waiting for UI thread..." );
 #ifndef Q_WS_MAC
     vlc_join (p_sys->thread, NULL);
 #endif

modules/gui/qt4/util/qvlcapp.hpp

@@ -40,10 +40,17 @@ class QVLCApp : public QApplication
 {
     Q_OBJECT
 
+private slots:
+    void doQuit()
+    {
+        closeAllWindows();
+        quit();
+    }
+
 public:
     QVLCApp( int & argc, char ** argv ) : QApplication( argc, argv, true )
     {
-        connect( this, SIGNAL(quitSignal()), this, SLOT(quit()) );
+        connect( this, SIGNAL(quitSignal()), this, SLOT(doQuit()) );
     }
 
     static void triggerQuit()