Cocoa: Set window title after setting window flags if window has border

A QWindow should keep its title after removing and restoring the
frame, so set it if we are setting window flags that include a frame.

Task-number: QTBUG-32978

Change-Id: I0fe1b651eac05a210b06ec4f7f6fb78f2536834d
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -486,6 +486,9 @@ void QCocoaWindow::setWindowFlags(Qt::WindowFlags flags)
         [m_nsWindow setStyleMask:styleMask];
         [m_nsWindow setLevel:level];
         setWindowShadow(flags);
+        if (!(styleMask & NSBorderlessWindowMask)) {
+            setWindowTitle(window()->title());
+        }
     }
 
     m_windowFlags = flags;