EXT-7381 FIXED (Nothing happens after clicking on some verb buttons of 'My Inventory' side panel)

- \"options_gear_btn\", \"trash_btn\" and \"add_btn\" now initialized using getChild of the main inventory panel (to avoid another break if the \"bottom_panel\"'s control class will be changed again from LLLayoutStack to something other, e. g. reverted back to LLPanel)
- mTrashButton member was added (which corresponds to \"trash_btn\" control) to optimize performance a little: LLPanelMainInventory::updateListCommands() updates \"trash_btn\"'s enabled state, so to avoid searching using getChild() again and again, a separate member was created.

Reviewed by Mike Antipov at https://codereview.productengine.com/secondlife/r/401/

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branch : product-engine