Symbian: Proper support for FEP input in QLineEdit with input mask

QLineEdit with input mask reports the cursor position relative to
displayed text via inputMethodQuery(), but the text returned is
the actual text of the control, which can differ from displayed text,
causing mismatch between FEP display and control display.

To properly fix this we would need to know the displayText of
QLineEdit instead of just the text, which on itself would be a trivial
change. The difficulties start when we need to commit the changes back
to the QLineEdit, which would have to be somehow able to handle
displayText, too.