Feature: Override name logic shared for backdrop containers

[Tilix4]

Changelog Description

Share logic for overriding name for all backdrop container based loaders (e.g TemplateLoader and PsdLoader)

Additional review information

Basically an extension of #95

Testing notes:

1. configure HarmonyLoadPlugins setting override_name to a bracketed template (e.g. {folder[name]}_{product[name]})
2. load a tpl representation and verify the loaded backdrop is renamed accordingly
3. load without override set; ensure default naming still works and no errors occur when no backdrops are initially selected