Use "@string" and "@Preamble" throughout.

Manual update: rewrite section "Exporting entries".

Also:

- Add News items to the manual.
- Update version number.

Update version number to 2.13.

When editing the crossref field, provide completion for keys in all d…

…atabases.

ebib-search-next: only search visible entries if database is filtered.

Update version number.

Look for the exact ID when opening a note.

If you have a key "Jones2012" and a key "Jones2012b", notes entries should be
filed correctly.

Take the year for display from the Date field if possible.

See Github issue joostkremers#120.

Update indentation for nameless-mode.

Always check if file on disk changed when saving a database.

The code that compares the modification time of the database with the modification
time of the file on disk is moved from `ebib-save-current-database' to
`ebib--save-database', which is also used by `ebib-save-all-databases' and `ebib-save-from-multiline-buffer'.