Re-add description for on Special:Version

Got killed when we removed the i18n stuff
Re-add Property::newEmpty for now
Update RELEASE-NOTES.md
Back to 0.7.3 description
Unchain SiteLinkList adder
Merge pull request #111 from wmde/unchain

Unchain SiteLinkList adders
Merge pull request #119 from wmde/description

Back to 0.7.3 description
Move diff classes into their own NS
Move diff code out of Entity and derivatives
Type hint against a more narrow interface
Add more fine grained tests for EntityDiffer
Add dedicated tests for ItemDiffer
Add getConstructionDiff and getDescriptionDiff to EntityDiffer
Minor style tweaks