Bug fix: do not add text properties beyond length of field value.

`ebib-get-field-value`: when adding the text properties `ebib--alias` and
`ebib--xref`, do not add them beyond the length of the string. If the string is
empty, adding text properties from 0 to 1 yields an error.

Thanks to Frank Schweitzer for reporting this bug.

Update version number to 2.22

Add warnings that changing `ebib-keywords-file` requires a restart of…

… Ebib.

This commit is in response to Github issue joostkremers#146. While not a true fix, it should
alleviate the problem.

Bug fix: ebib--notes-buffer: return the notes buffer upon first invoc…

…ation.

Add user option `ebib-default-directory`.

- This option sets the default directory for Ebib buffers.
- The DIR argument of `read-file-name` has been removed, so that the value of
  `ebib-default-directory` is the start directory for file names read from the
  user. This causes a slight regression for users that have customized
  `ebib-preload-bib-files`. They will need to set `ebib-default-dir`.

Update version number to 2.20.

Undo making biblatex the default dialect.

Fix regex in `ebib-browse-doi`.

Add biblatex citation commands to `ebib-citation-commands`.

Make biblatex the default dialect.