Calling the OGS File Converter as an external application

This adds a system call to open the OGS File Converter (FC) from the tools menu.
The FC is build as an OGS-5 tool to allow batch conversion between old and new OGS files. For the Windows version of the Data Explorer the FC is deployed with the downloadable zip and will be available in the working dir (so calling it will not be a problem).

If I am doing something wrong here, pls let me know.
Also, I don't know if this works via Linux.
An up-to-date version of the FC-utility is available from the OGS-5 trunk.