EXT-7984 FIXED (onVisibilityChange callback was used instead of onOpen)

Removed code from LLPanelOutfitEdit::onOpen and move it to LLPanelOutfitEdit::onVisibilityChange (as suggested by Nyx Linden at http://jira.secondlife.com/browse/EXT-7984).

Reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/694/

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branch : product-engine