Add project tree option to rename item to first heading

[vkbo]

Summary:

This is a simple PR that adds a context menu option "Rename to Heading" under the regular "Rename" option, if the selected item is a document, and there is a heading recorded by the index. When triggered, this option pre-populates the text field of the Edit Label pop-up with the heading text rather than the current item label. It uses entirely pre-existing functionality, so the PR simply adds the option and updates the unit test.

Related Issue(s):

Closes #1443

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed