Projects: Auto use {project_root} for asset library paths

Add a PROP_VARIABLES_PROJECT to indicate a string property can use project
variables, and use it for the asset library path. The file browser will
make such paths automatically project relative when writing back to the RNA
property.

To make it so this works for project asset libraries but has no effect
on preferences libraries, there are now two separate UserAssetLibrary
subclasses with and without PROP_VARIABLES_PROJECT on the path.

Pull Request: https://projects.blender.org/blender/blender/pulls/163777

scripts/startup/bl_operators/project.py

@@ -772,10 +772,6 @@ class PROJECT_OT_AssetLibraryAdd(Operator):
             .decode('utf-8', 'ignore') \
             or data_(ASSET_LIBRARY_DEFAULT_NAME)
 
-        # Replace base path with {project_root} if it is within the project folder.
-        root_path = bpy.data.project.root_path
-        if asset_library_path.startswith(root_path):
-            asset_library_path = "{project_root}" + asset_library_path[len(root_path):]
         bpy.data.project.asset_libraries.new(name=asset_library_name, directory=asset_library_path)
 
         return {'FINISHED'}