Define name property in Override

[nichenqin]

Which is the value of String(override.sketchObject.overridePoint().layerName()). I think it is useful if we can not get the origin id or can not ensure the order of instance.overrides

[mathieudutour]

the issue with layerName() is that it fall short when dealing with nested symbols and/or mutliple overrides points per layers (which might happen in a foreseeable future)