Add QAtomicIntLoad via new QAtomicIntCompat.h header.
Add QAtomicIntLoad via new QAtomicIntCompat.h header.

This commit adds a new compatibility header for
loading QAtomicInts portably between all supported
Qt 4 and Qt 5 versions.

The compatibility matrix is as follows:

Qt 4's QAtomicInt has a conversion operator (operator int())
for loading the atomic value.

In Qt 5.0 through 5.2, operator int() is not present, replaced
with a load() method.

However, Qt 5.3 introduces support for operator int() once again.
This means that Qt >= 5.3 support both .load() and operator int().

To overcome this problem without littering our code
with preprocessor ifs for QT_VERSION, we're introducing
a function, QAtomicIntLoad, that will perform the appropriate
load operation on a given Qt version.

This new function lives in QAtomicIntCompare.h.

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