adjust export-ignore checking in deleted files action

This makes use of git check-attr to properly check the attributes. The
attributes have been extended to match subdirs and files.

Automate the release workflow using GitHub actions

This introduces a two-part release mechanism. A manually triggered
workflow asks for the important info like type of release (stable, rc)
and code

Automate the release workflow using GitHub actions

This introduces a two-part release mechanism. A manually triggered
workflow asks for the important info like type of release (stable, rc)
and code name. It then creates a cleanly mergable pull request.

When that pull request is merged, a release is automatically tagged,
built and uploaded.

Another workflow is introduced to keep track of the deleted.files info.
This is one less chore to do on a release.

A new scheme for tags is also introduced, making all tags sortable,
regardless of their type. They follow the pattern

release-YYYY-MM-DD(<hotfixletter>|rc)

A script will be used to clean-up the existing tags.

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ignore the .github directory for exports

do not export the appveyor config

some more ignores

Use git attributes to exclude some files from exported archives

crlf 2 lf whitespace fixes