Properly test internal window flags

Qt::Popup is a mask, so we cannot use the `&` operator to test the window
type. We need to use QFlags::testFlag() for that purpose instead.

internal_client.cpp

@@ -213,7 +213,7 @@ bool InternalClient::isResizable() const
 
 bool InternalClient::isPlaceable() const
 {
-    return !(m_internalWindowFlags & Qt::BypassWindowManagerHint) && !(m_internalWindowFlags & Qt::Popup);
+    return !m_internalWindowFlags.testFlag(Qt::BypassWindowManagerHint) && !m_internalWindowFlags.testFlag(Qt::Popup);
 }
 
 bool InternalClient::noBorder() const