Log Message:
tex-tex4ebook{,-doc}: update to 0.3c

0.3b
New features:
- AZW and AZW3 support
- clean up of the generated files using make4ht DOM filters

Bugfixes:
- fixed XML validity issues for the NCX files

0.3c
Bug fix release: some custom elements remained in generated XML files in
the Epub 3 format, resulting in non-valid file.