PNG stitcher and unstitcher for groups of textures

This pull request adds the feature to import and export whole PNGs of groups of textures (Blocks, Items, GUI and Entities) as a single PNG file paired with a json. The file can then be modified with any editor and imported directly with the tool, any modified single sprite will be included in the selected texture pack.

The feature allows to edit whole texture packs by working on a single images, as it was before Minecraft switched to resource packs.