BibEdit: safety features, refactoring, bugfixes

• One cache per user per record. No more cache hijackings. Future diff and merge will be easier to implement.

• BibEdit deletes unneeded caches e.g., when a user leaves an unchanged record.

• "Freshness checks" - detect if DB record has been updated during the lifetime of the cache. Check when getting or submitting record. Propose various solutions to the user.

• Detect and handle if cache file disappears unexpectedly.

• Detect and handle if cache file is updated outside the active editor (by BibEdit in other window, or other tool e.g., BibMerge).

• Confirmation dialogs when submitting a record, deleting a record or cancelling editing

• Whenever possible, warn user if he/she leaves unsubmitted changes behind.

• Submit is now disabled if record is unchanged, but activates and turns green when record gets edited.

• Fixed bug which made it possible to edit protected fields.

• Hotkeys are now properly ignored in situations where they would be inappropriate.

• Hash management somewhat simplified, by not trying to accomodate hashes invented by the user.

• Heavy refactoring...

• Many small bug fixes.