WAGettextExporter catalog generation issues

[jbrichau]

Couple of issues to be fixed, which either already existed or are newly introduced in the new implementation included in Seaside 3.4.x:

• The comment of each translation entry in the .pot (catalog) file exported by WAGettextExporter may contain a method multiple times if multiple translation strings are in the method (this was already the case). A method should be included only once.
• The package name included in each comment is the package name of the class, not that of the method. Moreover, since multiple methods can be listed, the package name of the only first method is shown. We can safely remove the package name from the comment since the class name is sufficient to identify the method location.
• The meta header was removed from the .pot (catalog) file in the reimplementation.
• There is no interface to restrict the extraction of strings to a subset of packages.