Use the AddFileExtListToFilter() to also generate the
wildcard for "all files". This is because:
* Users can use AddFileExtListToFilter for the all files WC
with the same interface as for any other extensions.
* Users do not need to worry about wxGetTranslation, as the
_() is applied in the same way as the other *Wildcard() helpers,
and it is a function just like the others, so it is consistent
* There is a testable interface to document the expected result.
The test is added.
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