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: Cleanup: Composite nodes: add namespace for every file for more information.