A very small and easy fix for resourcepack devs

Minecraft Chests, Ender Chests and Barrels use the same texture for their UI. It would be much better if they have their own UI Texture, such as the shulker box. This would allow resourcepack devs to change the UI of the different containers individually without them sharing the same texture.

This would be solved by simply adding a "ender_chest.png" and "barrel.png" to the gui container textures like the "shulker_box.png". "generic_54" could then be renamed to "chests.png" because only chests, double chests, chest minecarts and boat with chests use this texture.

I think that this change would be very well fitting into the updates right now because Mojang is currently reworking Minecraft's UI like removing the dirt background.

I would like to see this being implemented in one of the next snapshots  : )