FieldFilter::addFilter doesn't work as expected

When filter is added using addFilter, during the visitTree it's not respected in the way it does when created in the beginning. However "description" works correctly.

Once fixed, it will allow us:

1. be confident and more flexible when using Filters while traversing gigantic DOMTree
2. Remove unnecessary tedious specification of fields like the one here: https://codereview.qt-project.org/c/qt/qtdeclarative/+/575407/44/src/qmlls/documentSymbolSupport/documentsymbolutils.cpp#83

Details:

https://codereview.qt-project.org/c/qt/qtdeclarative/+/566641/21/tests/manual/qmldom/dataModel/qmldomdatamodel.cpp#373