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.
Fixed broken Grid test.

It wasn't changed to take into account the new plug default values when we turned on GL line smoothing.
Fixed test failure in Bookmarks.acquire().
Merge pull request #765 from johnhaddon/compoundNumericPlugSerialisation

Combined setValue() serialisation for CompoundNumericPlugs