Add a very basic inventory panel

Add a very basic inventory panel to the game opened by pressing `i`,
which displays the contents of the player's inventory in a basic list.

src/Xanthous/Game/Lenses.hs

@@ -46,6 +46,7 @@ initialStateFromSeed seed =
       _messageHistory = mempty
       _revealedPositions = mempty
       _promptState = NoPrompt
+      _activePanel = Nothing
       _debugState = DebugState
         { _allRevealed = False
         }