version 2.1.1: compatibility with Thunderbird 68+; small presentation and documentation improvements

## Version 1.1:
* improve progress calculation and display; clean up photo image handling
## Version 1.0.9:
* fix bug introduced in version 1.0.8 regarding manual selection which side to keep

Version 1.0.7:
* add option for normalizing international call prefix
* fix horizontal layout issues, automatic width of contents
* improve name matching: allow substrings, stop removing singleton digits and letters
* mail user names like no-reply@... or no.service@... not anymore taken as first+last names

Version 1.0.6: various UI layout (width, vertical scrolling) and small documentation improvements

correction of mistake in packaging version 1.0.4 that prevented it from running

Fixed German (de) locale that disabled the extension for German version of Thunderbird.
Minor further locale/translation improvements.

Version 1.0.1 and 1.0.2:
- improved label of DCM menu entry for address book window
Version 1.0:
- major speedup in particular when searching for duplicates in large address books
- improved user guidance; new Tools menu entry with default address book selection
- various improvements of content matching and card comparison for equivalence
- cards may be excluded from being presented as matching by setting a different AIM name
- photos are compared for equality and are shown during manual inspection
- mailing list membership is taken into account for comparison and shown during inspection
- during manual inspection, field-by-field (resp. set-by-set) comparison information is shown
- option to consider phone numbers with national prefix and with default country code equivalent
- option to customize list of ignored fields; output summary of different fields
- option to preserve entries of first address book when auto-deleting redundant entries
- options are saved in TB configuration/preferences at extensions.DuplicateContactsManager.*