GUI: Scroll to correct datasheet position after deleting/inserting variables.
* src/ui/gui/psppire-data-sheet.h (psppire_data_sheet_delete_variables,
psppire_data_sheet_insert_new_variable_at_posn): New functions
* src/ui/gui/psppire-data-sheet.c: insert_new_variable ->
psppire_data_sheet_insert_new_variable_at_posn.
* src/ui/gui/psppire-data-sheet.c: delete_variables ->
psppire_data_sheet_delete_variables.
* src/ui/gui/psppire-data-editor.c:
(psppire_data_editor_data_delete_variables,
psppire_data_editor_insert_new_variable_at_posn): Reimplement.