Option to use pre-Jappa textures (for mobs that existed before 1.14)

[EvModder]

In 1.14, Minecraft redid all the mob models/textures.
It might be interesting to have a config option (disabled by default) to use the "classic" textures for pre-1.14 mob heads.

[EvModder]

Status update:
This is mostly done, for all the common mob types at least (those which do not have a large number of sub-variants)

Important Note:
Some head still don't have a 1.14+ "post-JAPPA" version yet -- I updated the ones I could, but will need to find a skin artists for some of the more complex mob models.

Once these heads have an updated texture, then their current texture will be moved to the category of PRE_JAPPA (which is the label being used to identify these textures in head-textures.txt)