Fixed unwanted copy/paste Plug.setValue() calls.

When the input to a plug wasn't part of the selection being copied, we were computing the input value and storing it as a setValue() call in the serialisation. This was bogus, because any computed value can be dependent on any part of a Context, and the Context in which we perform the serialisation may not be suitable for computing the value. Now copy/paste with excluded connections causes the plug to revert to its default value, which is more like the disconnection behaviour.

There is an argument that we should actually be outputting a setValue() call, but for the value that would exist if the connection was removed, but for now we've decided that this approach is sufficient to fix #740.

As for a fuller fix, there are still some open questions about how disconnection should affect the plug anyway. Perhaps for plugs with inputs which aren't from a ComputeNode (just a direct connection from a promoted plug for instance), there is an argument that they should retain the connected value rather than revert to their previous value. When we resolve these questions we could also revisit this issue.
Removed FormatPlugSerialiser from the public interface.

FormatPlug isn't intended for use as a base class, so there is no need to expose the FormatPlugSerialiser for subclassing.
Refactored FormatPlugSerialiser to subclass ValuePlugSerialiser.

This allows us to remove a bunch of code which appears just to have been copy pasted a long time ago, since when we have made changes to the original. Also removed comment which appeared not to reflect the code at all, and added a todo to find out why we need to serialise all the registerFormat() stuff anyway.
Tidied up GafferImage::Format bindings.

- Fixed include guards (GAFFERIMAGE instead of GAFFER)
- Removed FormatBindings namespace
	- Introducing a new namespace for just one function isn't really in the Gaffer style
- Made formatRepr take a reference rather than a pointer
	- It's not an optional argument, and Format variables are rarely accessed via pointer
- Removed unused includes
- Put internal stuff in an anonymous namespace
- Fixed inaccurate namespace comment
Merge pull request #748 from johnhaddon/copyPasteValues

Copy paste fix