SkinTemplate: Move $stylename to Skin and soft-deprecate

It is *used* by Skin, so let's define and document it there as well.

Also, it was never necessary to set this to anything, but we were
pretending it was. This only served to increase the amount of magic
skin creators have to accept.

Change-Id: I61c6e952ede72bad37db38fefe1299da798914b8