ContextVariablesTweaksUI : Start plug names at tweak0

The `name` being passed to `__addTweak()` is always empty, so we never hit the original naming code, and ended up with the first plug being called `tweak` and the second being called `tweak1`. Starting with `tweak0` matches us up with the other tweaks nodes.

python/GafferUI/ContextVariableTweaksUI.py

@@ -176,8 +176,7 @@ def __addTweak( self, name, plugTypeOrValue ) :
 		else :
 			plug = Gaffer.TweakPlug( name, plugTypeOrValue() )
 
-		if name :
-			plug.setName( name.replace( ':', '_' ) )
+		plug.setName( "tweak0" )
 
 		with Gaffer.UndoScope( self.getPlug().ancestor( Gaffer.ScriptNode ) ) :
 			self.getPlug().addChild( plug )