Added public Serialisation::acquireSerialiser() method.

This will allow more flexible serialisations to be made.
Added ValuePlugSerialiser::valueNeedsSerialisation() method.

This can be reimplemented by derived classes to provide more control over the serialisation of values.
Combined setValue() serialisation for CompoundNumericPlugs.

If none of the child plugs have a connection, then we can simply serialise the value with a single setValue() call on the parent.

Fixes #761.
Merge pull request #765 from johnhaddon/compoundNumericPlugSerialisation

Combined setValue() serialisation for CompoundNumericPlugs