handlr does not respect desktop file IDs from subdirectories

[Celti]

According to the Desktop Entry Specification, desktop file IDs do not map one-to-one with filenames; desktop file IDs for entries in subdirectories of the appropriate data directory include those parent directories with directory separators replaced by dashes; e.g., .local/share/applications/myapp/plugins/foo.desktop maps to myapp-plugins-foo.desktop.

This is handled properly in GNOME and elsewhere but handlr requires me to specify myapp/plugins/foo.desktop.