Currently, most node buttons are defined in `drawnode.cc` however,
this is inconvenient because it requires editing many files when adding new nodes.
The goal is to minimize the number of files needed to add or update a node.
This commit moves most of the node layout functions for composite nodes into their respected
`source/blender/nodes/composite/nodes` file.
In the future, these functions will be simplified to `node_layout` once files have their own namespace.
See {D13466} for more information.
Reviewed By: HooglyBoogly
Differential Revision: https://developer.blender.org/D13523
4e98d974b5