QGraphicsAnchorLayout: remove a misguided use of QLinkedList

QSimplexConstraints are held in QList everywhere, yet one
single function, getGraphParts(), used a temporary
QLinkedList. It did so because the function repeatedly
walks the list, erasing elements from it until no more
elements have been removed.

Thus, in O-terms, QLinkedList is the correct choice here.
Sadly, O-notation completely ignores the per-element cost,
and this is where QLinkedList suffers. By the time a QList
has shifted all of its elements left once, the QLinkedList
probably has just finished allocating its first node.

So, use a QList instead.

That, however, turns the it = erase(it) loop quadratic, so
re-formulate the processing part as a lambda and use
std::remove_if. Don't even erase until we know how many
items to erase.

As a benefit, we save the final conversion of the remaining
items back to a QList, and we can use QList::op+ to build
the initial list, reducing the number of allocations
performed by that container to one.

Also saves ~770b in text size on optimized GCC 5.3 Linux
AMD64 builds.

Change-Id: Iecf9e7961dd2b6b20039b9b0d472e32b3fae6994
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

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