qaxbase: Make sure of null termination in sigs, propsigs and props maps

In preparation for QT_ENABLE_STRICT_MODE.

As the implicit conversion to char * is disabled in this mode, we have
to make sure that everything behaves. Make sure that only null
terminated byte arrays are entered into the QMaps. This might be
overprotective but should not cost much.

In doing so we can rely on constData whenever the values from these byte
arrays are used for lookups as char arrays.

The interface was changed because addSignal and addProperty are
exclusively called using QByteArrays. Instead of hardening every calling
site we now make sure that these byte arrays are null terminated inside
the functions.

Task-number: QTBUG-145279
Pick-to: 6.12 6.11
Change-Id: I337f888f9e24f293572e651a5fc9b8e1442474fd
Reviewed-by: Marc Mutz <marc.mutz@qt.io>