QDirModel: replace a QStack with a std::stack

std::stack, based on std::vector, tends to be more efficient that
QStack, based on QVector, but, crucially, it is an actual stack,
so doesn't allow, say, iteration or insertion in the middle, which
QStack does allow.

Since a lot of places where QStack is used use at least some of
those non-stack features, protect this instance which doesn't, yet,
from becoming another such site.

Since the use of std::stack with the default std::deque increases
text size by almost 1K, and a vector makes a perfectly good stack,
use std::vector as the backing container (which std::stack, being
a container adapter, allows easily).

Saves ~0.5KiB in text size on optimized GCC 5.3 Linux AMD64 builds.

Change-Id: I51c8fb1dc4e9907ae00ed1cee8f320304321b322
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

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