Add QVarLengthArray::{indexOf,lastIndexOf,contains} functions

[ChangeLog][QtCore][QVarLengthArray] Added the indexOf, lastIndexOf and
contains functions to QVarLengthArray.  These functions make the class
more similar to QVector.

Change-Id: I9bd2b22bd8b7151c2d17aede36e5f2126570600b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>