Avoid the GIL in SignalManager::retrieveMetaObject

After massive GIL savings in the generated code, there still
exists a place where a lot of repeated GIL acquirements are
done. It was observed that up to 24 consecutive calls to
retrieveMetaObject(self) were made, all with the same value
for 'self'.

The code in question was:

(1)    Shiboken::GilState gil;
(2)    MetaObjectBuilder *builder = metaBuilderFromDict( \
                   reinterpret_cast<SbkObject *>(self)->ob_dict);
(3)    if (!builder)
(4)        builder = &(retrieveTypeUserData(self)->mo);
(5)
(6)    return builder->update();

An exact analysis of the code showed that the GIL usage (1) could
be moved out of the function into a deeper function that does
updates in a branch (6).

Function retrieveTypeUserData does not involve the Python
interpreter at all (4).

It took some time until it was proven that access to some
special Python dictionary cannot reach the Python interpreter
and therefore does not need the GIL as well (2).

This replaces the abandoned effort to write a "Lazy GIL".
It worked great for the example program, but had problems
with some never finishing tests.

After all, this solution is much simpler and works perfectly
well. More effort seems not to be necessary to handle the GIL.

Task-number: PYSIDE-803
Change-Id: I439009ff933fc6f498beb0c7f1523b6f985afda8
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

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