Add support for querying an elided sub-string from QTextEngine.

Add support for querying an elided sub-string from QTextEngine.

The QML Text element attempts to layout text first without eliding and
only builds an elided string for the last visible line.  This allows
it to re-use the already generated layout information to build the
elided string rather than doing a second layout of the relevant text.

Change-Id: Ieee45902d0c129853abe822c465eece4d634a028
Reviewed-by: Martin Jones <martin.jones@nokia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>