Sign editing screen uses menu_background.png

The bug

The user interface for signs and hanging signs uses the file menu_background.png, which is inconsistent with all other blocks with a user interface.

Ideally, they should use a texture file for the background instead of being hardcoded (MC-268927), however not the same one as for game menus.

How to reproduce

1. Download and apply the attached resource pack
2. Open the Game Menu
3. Open a chest or your inventory
4. Place a new or edit an existing sign or hanging sign

How to fix

Introduce a new texture file, inventory_background.png, which is used for the background texture in the user interface of signs and hanging signs, as well as for any block or entity with an inventory.