EXT-7639 FIXED added shop button which reacts according to the ticket description

- Added method LLAccordionCtrl::getExpandedTab() which search for the first expanded accordion tab. This method is a bit of hacking, but I discussed it with Vadim Savchuk.
- LLAccordionCtrlTab::isExpanded() is made 'const'. This is necessary to call it from getExpandedTab() which is also 'const'.
- Added all provided Marketplace URLs (taken from EXT-7257) to the settings.xml. So URLs can be configured without recompilation.
- Added method LLCOFWearables::getSelectedItems() which can be used to get ALL selected items (not only one).
- Class LLShopURLDispatcher is implemented in the llpaneloutfitedit.cpp.
- Added callback LLPanelOutfitEdit::onShopButtonClicked() which calls LLShopURLDispatcher to send user to the correct URL.
- Added shop button (shop_btn) widget in panel_outfit_edit.xml. It has invalid icon for now. After review I'm going to reassign the ticket to someone who will provide correct icons.

Reviewed by Vadim Savchuk and Mike Antipov at https://codereview.productengine.com/secondlife/r/569/

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branch : product-engine