EXT-4003 Appearance problemw earing City Chic Female - probably param related

cross-wearable params are fun! Values were not being updated properly for
parameters that cross multiple wearables. Created a few functions to ensure
that these values get updated and made them called from
LLAgentWearables::wearableUpdated().

Also prevented cross-wearable params from writing back to the avatar, as they
are being driven by another wearable.

Code reviewed by Bigpapi