1. Use template to accommodate the new syntax of signal/slot connection.

2. Instead of the four parameters passed to QObject::connect(), we
store the QMetaObject::Connection returned by connect(), and use that
to disconnect() it afterwards.

3. There is a unit test to test the auto-(dis)connection, just to
make sure it works with both syntaxes.

4. There is no need to change any existing code involving
KisSignalAutoConnectionsStore, unless you want to replace the old
syntax with the new one.