allow to modify/replace artifact-listing contents at runtime

[x1B]

Currently, all application artifacts (widgets, pages, flows, layouts...) are prebundled using the laxar-loader for webpack.
Sometimes it is desirable to add or replace individual items, e.g. to load user-defined pages from a REST-service.

To implement this feature, we could allow to decorate the axArtifactsProvider through the application configuration (which would be created with a reference to the original provider).