qt4: fix skins2 crash after exiting the preferences dialog

This fixes a regression brought by 7f485998dd63c9685f0a470439c9b2ea127ef5c5

On skins2(tested on Linux), open and close the preferences dialog leads to a
crash, because the whole qt4 thread terminates unexpectedly.

This is due to setQuitOnLastWindowClosed set to true. As a dialog provider,
qt4 doesn't run any windows, and therefore just closing the preferences
 dialog box (via accept() or reject()) signals the qt4 thread to quit.
(cherry picked from commit 3326e7ef0e333e7a1aa6419c4547703dcbd23618)

modules/gui/qt4/qt4.cpp

@@ -454,7 +454,10 @@ static void *Thread( void *obj )
     vlc_sem_post (&ready);
 
     /* Last settings */
-    app.setQuitOnLastWindowClosed( true );
+    if( p_intf->p_sys->b_isDialogProvider )
+        app.setQuitOnLastWindowClosed( false );
+    else
+        app.setQuitOnLastWindowClosed( true );
 
     /* Retrieve last known path used in file browsing */
     p_intf->p_sys->filepath =