QVarLengthArray: C++11-ify insert/erase signatures

QVarLengthArray: C++11-ify insert/erase signatures

In C++11, container insert and erase operations take const_iterators
instead of iterators. This is a bug fix compared to C++98, where the
mere lookup step of a lookup-or-insert operation had to be done using
(mutable) iterators, which is particularly worrisome for Qt containers
that are implicitly shared, because of the unneeded detach in the positive
case.

QVarLengthArray is not implicitly shared, but for consistency, the signatures
should be changed here, too. The reason this commit contains only the change
to QVarLengthArray is that this is by far the easiest container. The
implictly shared containers are harder, because detaching invalidates other
iterators (more than the sister STL container would).

Change-Id: Ib3d98360bfe376b782b9d1283c5fa3555e8a719e
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>