gui: wizard: importdata: Make preview data accessible earlier
gui: wizard: importdata: PreviewPage: Add javadoc
gui: wizard: importdata: ColumnPage: Add text to first column
gui: wizard: importdata: ImportDataColumn: Add index of column
gui: wizard: importdata: ImportData: Implement getPreviewDataForColumn()
gui: wizard: importdata: ColumnPage: Full support for datatype choosing