Added accessors for the buttons on VectorDataWidget.

Here's an example config file using the new accessors to do some cheeky remapping of paths that are dropped into the PathFilter UI.

```
import GafferUI
import GafferScene

def __pathsDrop( widget, event ) :
	
	# monkey with the data
	for i in range( 0, len( event.data ) ) :
		event.data[i] = "/prefix" + event.data[i]
	
	# defer to the original drop handler
	return False

def __pathsPlugWidgetCreator( plug ) :

	result = GafferUI.VectorDataPlugValueWidget( plug )
	result.vectorDataWidget().setDragPointer( "objects" )

	# connect in front of the default drop handlers
	vdw = result.vectorDataWidget()
	for w in vdw, vdw.addButton(), vdw.removeButton() :
		w.__dropConnection = w.dropSignal().connect( 0, __pathsDrop )

	return result

GafferUI.PlugValueWidget.registerCreator(
	GafferScene.PathFilter,
	"paths",
	__pathsPlugWidgetCreator,
)
```