Rename the project from “lang-table” to “langtable”

A “-” does not work in the name of a Python module, i.e. lang-table.py
does not work. Therefore, I used “langtable.py”. But then it
is nicer if the .spec file and the the whole project is als called
just “langtable” for consistency.
Move the .xml files to a data/ folder in the sources