New undo methods for multiple headline changes on branch 'boltex/issue3219'

[Félix]

If you'd like to test out some new proposed methods to undo/redo commands that alter many headlines at once: https://github.com/leo-editor/leo-editor/tree/boltex/issue3219

Thanks for reviewing my code if you feel like it. Also, you can easily test it with the attached Leo file which provides 2 @buttons that try to change the headlines in the outline. One does it one by one, the other does it all at once (using the new proposed undo helpers):

Félix

[Thomas Passin]

Without a PR, it's hard to review the code without knowing what has been changed.

[Félix]

oh, you're right, https://github.com/leo-editor/leo-editor/pull/3228 is what i meant to paste