Use correct extension when loading model in SimpleBlockOBJRenderer

Signed-off-by: Steven Downer <grinch@outlook.com>
Refactor CustomBlocks/Item to SimpleBlock/Item

Signed-off-by: Steven Downer <grinch@outlook.com>
Cleanup SimpleBlocks/Items

- Added missing overridden methods
- Rearranged entries for getAddon/getIdentifier/getLocalizedName/getUnlocalizedName/getItemStackDisplayName/equals/hashCode to all be the last methods in their respective classes.
- Suppress compiler warnings
- Resolved lighting issues with slabs
- Resolved incorrect item drops for slabs
- Merged SimpleItemSlab into SimpleSlab
- Formatting changes

Signed-off-by: Steven Downer <grinch@outlook.com>