Don't propagate dataChanged upwards when we are not filtering.
Review Request #126789 - Created Jan. 18, 2016 and updated
This avoids expensive layoutChange signals from QSFPM. We can only be sure we are not filtering if filterAcceptRows isn't overridden, which we can check if we assume sub-classes have the Q_OBJECT macro set. This will however break sub-classes overriding filterAcceptRows without using the Q_OBJECT macro, and using their own filter criteria. However, this is a significant performance improvement in the common case of KRFPM being used for string-based searching in a tree view, and the search being unused.
Tested in GammaRay, where the layoutChanges are particularly painful as the source model is in a different process.