EXT-7565 FIXED Would be nice to preserve selection while switching between folder/list view modes

- Created callback called 'SaveSelecton' (which preserves selection while switching between folder/list view modes) for folder/list view modes buttons in LLPanelOutfitEdit.

- Added 'scrollToShowFirstSelectedItem' method in the LLFlatListView which scrools and shows the first selected item in case multiselection.

- It's possible to set selection for flat list view items before list is build. The result is that any items will be selected. To get rid of it:
   1. Overrided LLFlatListView::selectItemByValue method in LLInventoryItemsList so that if list is not created yet, items ids are saved to the vector.
   2. Added 'LLInventoryItemsList::updateSelection()' method which selects items with ids from that vector when list is created.

- A little refactoring: moved funcionality of updating WearablesPanel's verb buttons to the separate method called LLPanelOutfitEdit::updateWearablesPanelVerbButtons() to made code more readable and self-explanatory

Reviewed by Vadim Savchuk and Neal Orman at https://codereview.productengine.com/secondlife/r/579/

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branch : product-engine