Add as_utils_is_blacklisted_id()

This moves the data to libappstream-glib where we can use it to prevent parsing
desktop files that should not be shown in the software center.
trivial: Supress warning when either desktop or appdata paths have subfolders
Add as_app_source_kind_to_string()
trivial: Log why a desktop file is being ignored
trivial: Show the source kinds to allow debugging duplicate entries