Follow-up, use set/getExtensionData() for MW 1.21+ in SMWFactbox

This follows [1], SMW\ParserTextProcessor uses set/getExtensionData()
for MW 1.21+ and for all others use mSMWMagicWords

[1] https://gerrit.wikimedia.org/r/#/c/60393

Change-Id: I4694e7b010e06e72dc53161cc9139f40e5bffd4f
Merge "Follow-up, use set/getExtensionData() for MW 1.21+ in SMWFactbox"