qmlprojectexporter fails to create translation files

When using multiple languages in a code base, the directive is to populate the `TranslationFiles.files` property with a list of translation files which correspond to the languages supported by the application.

There are three issues with this:

• If the *.ts files do not already exist, qmlprojectexporter improperly invokes lupdate and the *.ts files are not generated. Thus, cause a build failure.
• If the *.ts files do exist, but are empty, lupdate fails citing a premature end of the file.
• If the *.ts files do exist and have been previously generated by lupdate, the update_translation build target fails. Again, this appears to be improper invocation by qmlprojectexporter