EXT-6726 WIP Added more menus to the Appearance SP.

Done:
- Implemented creating new wearables via My Outfits gear menu.
- Implemented renaming/removing/editing an outfit via My Outfits context menu.
- Implemented "Attach to..." / "Attach to HUD..." context submenus.

- Now disabling (instead of hiding) irrelevant wearable context menu items.
- Added "Take Off / Detach" context menu item that's shown for clothes and attachments.
  Useful if you selected a bunch of items and want to take them all off.

- Fixed taking off an outfit
  (not all wearables were taken off because of a wrong inventory collector).
- Fixed crash when editing a skirt
  (reference to a missing string).

- In LLWearableItemsList::ContextMenu::updateItemsVisibility renamed variables
  and introduced MASK_UNKNOWN per Nyx's request.

Known issues:
- "Attach to..." context menus may be displayed partially off-screen
  (there is the same bug in the inventory panel).
- The way we invoke wearable editing panel after the wearable gets created
  is currently a hack.

TODO:
- Wear / take off / rename / delete an outfit via My Outfits gear menu
  (currently not implemented because of missing selection support in My Outfits).
- Add "Create new..." to body part / clothing context menu in Edit Outfit.
- Add "Create new..." submenus to the Edit Outfit gear menu.

Reviewed by Nyx at https://codereview.productengine.com/secondlife/r/466/

--HG--
branch : product-engine