Add remaining custom painting item sprites #132

@Ampflower has blessed our creative (or VT Datapack '') inventories with cute little icons for each and every painting we change. We should expand on that and add textures for the remaining items.

Here are the paintings with no item texture:

• Non-Human Unity Flag
• FOUR ××××××× PIXELS
• boat
• cave
• happy happens
• palm

This issue can be reopened when relevant (I think)