TextInput::displayText: include partial input from an input method

[ChangeLog][QtQuick][Important Behavior Changes] TextInput::displayText
now includes also partial input from an input method and thus matches with
the actual displayed text.

Task-number: QTBUG-40329
Change-Id: I407f464938c550d73eba1351283ec751aa293380
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>