Support asset mapping

[johnspackman]

The generator supported asset-let, which was a way of mapping assets at runtime; this is not implemented, but should be - and should include support for mapping to icon fonts

[hkollmann]

description is here:
https://qooxdoo.readthedocs.io/en/latest/pages/tool/generator/generator_config_articles.html#pages-tool-generator-generator-config-articles-asset-let-key