DEV-34977 Context menu fails to open in Build mode. Product Engine...

DEV-34977 Context menu fails to open in Build mode.  Product Engine implemented context menus as spawning on right-mouse-up, which interacts poorly with LLToolPie and recycled pie menu code.  Changed context menu spawn to right-mouse-down, which allows click-drag-select behavior like the Mac.  Eliminated unnecessary LLContextMenu::handleMouseUp and handleMouseDown methods -- these are handled in the LLMenuGL base class.  Added handleRightMouseUp to LLMenuHolderGL to hide menus on right-mouse-up outside the menu.  Reviewed with Richard.