Vector: Rename description message: vector-desc → vector-skin-desc

The former was also used by the old Vector extension and the
translations now conflict.

Intentionally didn't change qqq as it currently contains the docs for
the wrong message… I'll fix qqq on translatewiki.

Change-Id: Id7bde76b8f663e88672248f6881530ece54b67b1
Merge "Vector: Rename description message: vector-desc → vector-skin-desc"