Initial, limited implementation of loading a single asset catalog definition file. These files are structured as follows: CATALOG_ID virtual/path/of/catalog SUBCATALOG_ID virtual/path/of/catalog/child SOMETHING_ELSE some/unrelated/hierarchy These virtual paths will be used to show the catalog in a tree structure; the tree structure itself is not part of this commit. Each asset will have one catalog ID that determines where in that tree the asset shows up. Currently only a single catalog definition file can be read; merging data from multiple such files, and writing them out again after changes are made, is for future commits. This commit only contains the code to load a single file, and unittests to check that this actually works. No UI, no user-facing functionality yet.