Test: export options

[chlulei]

This PR reimplements the non standard export options of Test and TestQuestionPool to conform with the new export infrastructure. This also includes changes to the ilTestExportGUI and ilObjQuestionPoolGUI. For details see the documentation.

The class ilTestExportTableGUI is no longer in use, instead the Test export tab uses the Export's standard implementation with a new KS DataTable.

The export options for Test are "ARC", "XML", "XMLResults", "Plugin", and for TestQuestionPool "XML" and "XLSX". The implementation is a bit more involved because of file handling, but mainly consists of reshuffled previously existing code. An exception to this is the export option "Plugin" for Test. It is no longer possible to dynamically adapt the available export options according to what plugins are installed, so the export process now has a second step where the plugin is selected.