EXT-4846 : [NUX] Inventory Side Panel - Can we suppress the "Landmarks" and...

EXT-4846 : [NUX] Inventory Side Panel - Can we suppress the "Landmarks" and "My FavoriteS" folder since they have their own Task Panel?

Adding simple infrastructure for supporting ability to hide folder types from inventory panels, but NOT exercising this code.  This will be deferred for 2.1.
Also did some very superficial header file cleanup for LLInventoryPanel.