minor bug fixes/feature changes:

- activate "Recode"-menu only if database is open
- add ID (primary key) to entries in meta-list
- add yes/no-dialog to delete all-button (entries in meta-list)
- add yes/no-dialog to go-button
- add yes/no-dialog to cancel-button
- added .txt-extension to export-file with meta-list entries
- fixed bug with import txt-file for meta-list entries
- import meta-list: do not show "List of meta-entries..." or empty rows
- Recode-table: deleted horizontal scrollbar (not needed)