EXT-7963 FIXED (Edit Outfit > Add More > Do not switch to next item type after add/replace)

- Added selected item type (in flat list view) as criterion when determining filter type in 'Add More' panel

- Fixed LLAccordionCtrl::getSelectedTab() method. When 'selection_enabled = false' for LLAccordionCtrlTab, LLAccordionCtrl::getSelectedTab() returned NULL, even if some accordion tab was selected. Now it's OK. Method returns currently selected LLAccordionCtrlTab.

Recovered from bad merge in 13811

Reviewed by Richard Nelson at https://codereview.productengine.com/secondlife/r/790/

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branch : product-engine