Thunderbird 91 compatibility update

• Update WindowListener API

• Lightning is now always true

• Remove entry from Tools menu

The Tools menu is mostly not visible anymore and add-on options should all be located within the add-on manager (only).

• Raise compatibility to TB91

• Remove overlay.js, as it is not used.

• Fix whitespaces.

• Unload only if add-on is disabled/removed, not on app shutdown

• Bump version number.

• fix save (must return a Promise now)

• replace openAttachment by launchDownload()

launchDownload is used by the WebExt API downloads.open, it is able to open local files. openAtachment fails for most file:// links in TB91.

• Bump version number.