Combine multiple targets into single translation

qt_add_translations() only takes source files from one target. In more complex projects there could be several QML modules too, which due to limitations need to be separate targets. This means one has to either explicitly list every file or do qt_add_translations calls for each of those QML modules as well as the application and this also results in multiple .ts files.

Perhaps qt_add_translation() could be extended to also take additional targets from which it pulls files to run lupdate against and generates only one set of .ts files containing translations for all selected modules.