Inventory UI: the UI for the inventory

[culacant]

Show the actual UI in-game

For the inventory we will probably need the following:

• ShowInventoryBG() < would fetch the inventory background (TBD) and show it on the background
• ShowInventoryItemUnselected(int id) < shows a bitmap for item #Id, unselected
• ShowInventoryItemSelected(int id) < shows a bitmap for item #Id, selected (it's a different GXA file)
• InspectInventoryItem(int id) < shows a 3D model of the item

Identified the following relevant GXA files:

• INVBORDER.GXA: background graphic for the inventory
• INVSEL.GXA: background highlight to be shown behind the selected item
• INVARROW.GXA: arrows for scrolling up and down; frame 1 is uparrow, frame 2 downarrow
  along with the item icons, see ITEMS.INI

INVMASK.GXA exists, no idea what it is for, does not show up in redguard mod manager