Inspired by Unreal Blueprint's drag link+create list functionality. I
realize Blender has a few operators that do different parts of this job,
but they don't provide this well-polished functionality.
Version 2 of the python file node is now backed by a `bpy.types.Text`
datablock in the case of a file whose attributes are updated from a
backing file.
Parses and loads the PFM arguments into accessible properties, used
by numeric nodes for range, dropdown nodes for enums, etc.
Also moves PlasmaAttribDropDownListNode into its proper alphabetical
position.
This has some simple attribute nodes for example purposes. Unfortunately,
this is so far UI only. More node types need to be added, then we can
begin working on exporting.