Planner examples: import files should be filtered by files that can be imported

Especially in the machine reassignement example this is annoying, because when you click the "import" button there, it shows "assignement." and "model." files, but only model files can be succesfully imported.

The trick is to use a filefilter.

Howto:

Create the method SolutionBusiness.acceptImportFile()
which delegates to AbstractSolutionImporter.acceptInputFile().

Next, in SolverAndPersistenceFrame.ImportAction.actionPerformed(...) set a FileFilter and make it use SolutionBusiness.acceptImportFile().