Modeled similar to ConfigFactory, this lets skins
register themselves via a callback, allowing for
proper dependency injection.
Loading via $wgValidSkinNames is still supported,
but considered "legacy", not deprecated though.
Skin::newFromKey is now deprecated (and had only
one caller in an extension, which I'll update
afterwards).
Change-Id: I1960483f87c2ef55c994545239b728fa376f17f4
