ui_preview_setup

Setup environment for UI preview scripts.

Makes sure that 'ayon_core' and 'qtmaterialsymbols' are in sys path.

Also makes sure module 'ui_preview' is available. That is done by using a dummy module class if the module is not found. Python will auto-populate rest of the submodules if 'ui_preview' is available.

It might be possible that it does not work in all cases and we might need to import each file in 'ui_preview' package manually. But for now this should be enough.