QKeySequenceEdit: If the next key is Key_unknown then it should be ignored

When triggering a combination of keys which is causing a dead key then
it will send Key_unknown which is not a valid key to be used in a
shortcut so it should just skip past it as if it were a modifier key.

Task-number: QTBUG-57932
Change-Id: I16e004b84f3aa854f8f8f2bbdf86beb6d764de48
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/widgets/widgets/qkeysequenceedit.cpp

@@ -257,7 +257,8 @@ void QKeySequenceEdit::keyPressEvent(QKeyEvent *e)
     if (nextKey == Qt::Key_Control
             || nextKey == Qt::Key_Shift
             || nextKey == Qt::Key_Meta
-            || nextKey == Qt::Key_Alt) {
+            || nextKey == Qt::Key_Alt
+            || nextKey == Qt::Key_unknown) {
         return;
     }