VectorDataPlugValueWidget : Single value drop

Previously, we were letting `PlugValueWidget` attempt to convert dropped
data into something the target plug can support. When dropping a single
value such as `IntData` onto a `VectorDataPlugValueWidget`, this meant
converting `IntData` to `IntVectorData`. `*VectorData` supports
construction from an integer value, in which case it creates n elements
in the vector.

This commit instead converts a non-sequence value into a single item
sequence holding that value to use as drop data.

Changes.md

@@ -4,7 +4,7 @@
 Fixes
 -----
 
-- VectorDataWidget : Fixed bug preventing dropping a single value onto the `+` and `-` buttons.
+- VectorDataWidget, VectorDataPlugValueWidget : Fixed bug preventing dropping a single value onto the `+` and `-` buttons and the plug name.
 
 1.4.15.5 (relative to 1.4.15.4)
 ========

python/GafferUI/VectorDataPlugValueWidget.py

@@ -107,6 +107,17 @@ def _updateFromEditable( self ) :
 
 		self.__dataWidget.setEditable( self._editable() )
 
+	def _convertValue( self, value ) :
+
+		plugValueType = type( self.getPlug().defaultValue() )
+		if hasattr( value, "value" ) and isinstance(
+			value.value,
+			IECore.DataTraits.valueTypeFromSequenceType( plugValueType )
+		) :
+			return plugValueType( [ value.value ] )
+		else :
+			return GafferUI.PlugValueWidget._convertValue( self, value )
+
 	def __dataPlugs( self ) :
 
 		if not len( self.getPlug() ) :