ContextAdapter: Fix setting properties.

The mutex was actually causing too many return -1 and this corrupted
the properties. After that when they were passed to signal they
were often invalid because ::WriteProperty internally also emit
the signa, which also calls qt_metacall.

Note that signals are still not fully working, but at least it's
better then the previous commit.

The QModelIndexBinder has also re-enabled setProperty and I fixed
a few recursive loops that would undo all changes as soon as they
were done.