Ensure editor content is saved before using it

[vkbo]

Summary:

This PR:

• Adds a method to the shared object that can be called when it is important that the content of the document editor is saved before it is used to perform an action.
• Use this method in the Manuscript tool (replaces the old implementation).
• Adds this to the "Rename to Heading" context menu handler in the project tree.

Related Issue(s):

Closes #1914

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