QKeyEvent should provide unmodifiedText() method

QtWebKit and QtWebEngine both perform translation of QKeyEvents to internal events of respective browser engine, and both engines assume that key event has a field named "unmodifiedText", which is

text that would have been generated by the keyboard if no modifiers were pressed (except for Shift); useful for shortcut (accelerator) key handling. Otherwise, same as text().

Particular case where it matters is handling access keys (see QTBUG-64300 and https://github.com/annulen/webkit/issues/594). On macOS key events with modifiers come without text, which breaks access key handling that relies on text (we use text() both as "normal" and "unmodified" text)