Core/kernel: Make some signals private.

Core/kernel: Make some signals private.

There are more opportunities in QtCore and the rest of Qt to make signals
private instead of public. This is a test-dart to see if there is any
reason not to do this.

It would be nice to make QObject::destroyed private, but as it has a
default argument it would be source incompatible to anyone connecting
to the SIGNAL(destroyed()) instead of SIGNAL(destroyed(QObject*)).

Currently the function-pointer-based connect syntax does not accept
a functor (or lambda) with a different number of arguments than the
signal. Olivier says a fix for that might come in 5.1, but for now
the qfiledialog2 test is changed to not use that anymore.

Also, the function pointer for a private signal can not be assigned to
a local variable, so the qmetamethod test is changed to not do so
anymore.

Change-Id: Iaf776b822f9ba364f2c184df0c6b23811da56e44
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>