QFontComboBox doesn't support proxy models

At the moment applying a proxy model to the model of QFontComboBox breaks the class.
This is only due to a questionable design choice that is easily resolved.

Instead of creating the model in QFontComboBox::QFontComboBox and then relying on qobject_cast<QStringListModel *>(q->model()) to retrieve it, store the model pointer inside QFontComboBoxPrivate.

This allows the final user to use QFontComboBox::model to retrieve the original model, set it as the source for a proxy and setting this proxy back as the model for the combo.

This also fixes QTBUG-34730 as at that point the user is free to subclass QSortFilterProxyModel to filter out the fonts (s)he doesn't want.