Tweak the handling of the LayoutEntity files created by sotn-assets

[bismurphy]

This changes the formatting of the generated layout entities in order to make the files (in my opinion) more readable. We do the following:

1. change the types in e_laydef to make the offsets make more sense and reduce casts
2. Break up e_layout into blocks to more easily see where the boundaries between each room's data are
3. Add comments between those boundaries with the offsets, to easily cross-reference e_laydef and e_layout
4. Implement reading of the EntityIDs enum so that entities are listed by ID in e_layout.c to make it easier to identify them
5. Include the overlay's .h file rather than common.h, to allow access to the EntityIDs enum.

Sounds like we want to make changes here to move this stuff up to the JSON, so I'll mark this as a draft while we determine the long-term desired functionality.

Also, just a heads-up that this is my first time working with Go, so I might have done some bad code practices.

[bismurphy]

Okay, I moved the logic to layout.go, so now the entity name makes it into the JSON during extraction rather than being swapped out when making e_layout.c. The e_layout.c file is identical to what it was in my initial version, but now the entity names make it into the JSON. Example json snippet from NZ0:

[sozud]

Seems reasonable to me but ought to be approved by Xeeynamo