Add additional inputs option for lupdate

Original report: https://github.com/qt-labs/vstools/issues/25

Is your feature request related to a problem? Please describe.
When generating translation files for multi-project solutions, it would be very useful to list multiple projects as source paths for the lupdate command.

Describe the solution you'd like
In Project Properties -> Qt Translation -> lupdate there should be a "additional sources" parameter. When the command resolves to lupdate [options] [source-file|path|@lst-file]... -ts ts-files|@lst-file the additional sources would be placed before the "-ts".

Describe alternatives you've considered
lupdate has the -I parameter for "include files" but these don't seem to work for including additionl sources.