Add support for xd to ipuz-convertor via the xdfile library, and expose
support in the Crosswords UI.

xdfile states it depends on the "crossword" library which pip tries to
get from PyPI. This conflicts as xdfile uses its own patched version.
For our uses of xdfile, we don't need any of its dependencies, so
installing with --no-deps fixes that issue.

Closes: #112