Now that \lstlistoflistings is supported by InsetTOC, we should use that
instead of ERT. I could have updated the file format and directly used an
InsetTOC instead, but I was too lazy.
The listings inset and include inset of type listings use two english terms
that are not localized yet: "Listing" for the caption and "Listings" for the
list of listings (not supported natively by LyX yet). The existing layout
translation mechanism has been extended to translate these terms as well:
1) Support [[stuff]] context in lib/layouttranslations
2) Support BabelPreamble and LangPreamble in InsetLayout
3) Use a InsetLayout for InsetInclude of type listings
4) Define BabelPreamble and LangPreamble in the layouts for InsetInclude
and InsetListings
