Fix commented obsolete strings

the current implementation of items marked obsolete did not allow "plain"
comments for these items. However, this is perfectly fine according to
the original gettext tools. When writing a po file, comments for obsolete
items don't contain the '#~ ' mark (tested using msgcat), so this is now
also aligned with the behaviour of the original gettext tools.

For all these cases I added examples in the po files, that failed with the
current implementation and work fine after these changes.