Moved MetadaBinding implementation into unnamed namespace.
Removed custom TypeId argument handling from Metadata binding.

This is now taken care of for us by Cortex.
Made UI registration methods accept classes in place of TypeIds.

This matches the less verbose form we've always used for registering Metadata, and the form that is now available for all C++ registrations thanks to a new converter in Cortex.
Omitted staticTypeId() calls wherever possible.

This includes the following places :

- Factory registrations for PlugValueWidget
- Factory registrations for NodeUI
- Factory registrations for NodeToolbar
- Factory registrations for ParameterValueWidget
- Calls to GraphComponent.ancestor()
- Calls to GraphComponent.children()
- Calls to GraphComponent.commonAncestor()

This makes for more readable, more pythonic code.
Merge pull request #861 from johnhaddon/typeIdBinding

Removed unnecessary staticTypeId() calls