Three PRs merged into devel

[Edward K. Ream]

The devel branch now contains all recent code changes.

PR #3424 contains tweaks to Leo's unl code.

- leoSettings.leo now specifies gnx-based unls and short file names.

- The first comment of the PR contains a compatibility guide.

- The PR fixes bugs in the all-important g.openUNLFile function.

- leo/test/test.leo contains unls I used for manual testing.

  Leo's unit tests also cover the new unl code, but I use manual testing as an additional check.

PR #3433 fixes a small problem in Leo's refresh-from-disk command.

The code review revealed unused positional arguments in two helper methods.

After much discussion, I have chosen to remove those arguments.

There is a small chance that changing the signatures might impact existing code.

The release notes document those "breaking" api changes.

PR #3436 Improves the documentation for four unl-related methods.

Note: The docstring for g.findGnx resolves #3431.

Summary

The three PRs contain the last significant changes scheduled for Leo 6.7.4.

Please keep testing the devel branch!

Edward