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/Arbitrary.hs

@@ -26,6 +26,7 @@ instance Arbitrary GameState where
     _revealedPositions <- fmap setFromList . sublistOf $ EntityMap.positions _entities
     _randomGen <- mkStdGen <$> arbitrary
     let _promptState = NoPrompt -- TODO
+    _activePanel <- arbitrary
     _debugState <- arbitrary
     pure $ GameState {..}