Don't mix iterator and const_iterator

This avoids unnecessary detaches of the Qt container data.
The mismatches where detected by defining QT_STRICT_ITERATORS;
however, this define violates the ODR (causing linker errors),
and therefore is not added permanently.

Change-Id: Idd336a9c8b394214a820437ef1b92d2101f6101c
GPush-Base: 62b0848b
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>