bog-citekeys-in-file: Don't load Org mode
bog-heading-citekeys-in-file: Remove unneeded setq
Add command to list orphan citekeys
bog-heading-citekeys-in-buffer: Fix docstring
Add function to get citekeys in buffer
Prefer 'citekeys' to 'refs' as variable name
Support creating combined bib from multiple files

Before, bog-create-combined-bib only collected citekeys from a single
buffer.  Extend it so that it also works with marked files in Dired.

In addition

- Rename combined bib buffer so that it includes "Bog" in the name.

- Offer to skip missing bib files instead of failing.

- Remove bog-collect-references and bog-collect-unique-references, which
  overlap with the newer bog-citekeys-in-buffer [1] and should have been
  removed when that function was added.

[1] ce74785920d6234072eb2ebef63bd140a25534c2
Extend goto functions to support citekey properties

bog-goto-citekey-heading-in-buffer gives preference to citekeys as
heading titles over citekeys as properties.

The order of preference for bog-goto-citekey-heading-in-notes is the
citekey as a heading title in the current buffer, the citekey as a
property in the current buffer, the citekey as heading title in notes,
and, finally, the citekey as property in notes.