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 shader nodes into their respected
source/blender/nodes/shader/nodes file.
In the future, these functions will be simplified to node_layout.
Some nodes were left in `drawnode.cc` as this would require duplicating code
while this is likely fine it is best to leave that to a seperate commit.
e17ac8dc58