fix(menu/save-project): Add missing XML prolog on document save

[danyill]

Most of the DOM methods for detecting the declaration/prolog are deprecated (Document.xmlVersion, Document.xmlEncoding, DOMImplementation.hasFeature()) so I avoided using them. There do not appear to be replacements.

As per previous discussions on Save Project, I don't think we can add a test without modifying the code for the test.

I think we should leave the issue open until we have a test for this as part of the Action/Editing API. i.e. carry out action, check that the document is well-formed.

I would be grateful for testing/review as this is quite important.

Part of #1163

[ca-d]

LGTM, but is there a chance this might lead to prologue duplication in case there's a prologue with whitespace nodes before it or something (e.g. attribute order) is formatted differently? I don't know if these things can happen, or if there could ever be other encoding attribute values e.g.

[danyill]

LGTM, but is there a chance this might lead to prologue duplication in case there's a prologue with whitespace nodes before it or something (e.g. attribute order) is formatted differently? I don't know if these things can happen, or if there could ever be other encoding attribute values e.g.

I believe at best the document which did this would not be well-formed which is a reasonable requirement (?).

The spec requires that to be well formed the prolog should be unambiguously the first item (after the byte order mark): https://www.w3.org/TR/xml/#sec-prolog-dtd see section 2.1

[danyill]

Incidentally, it amuses me that we didn't notice this earlier. Anyone with a file editor GUI which inspected the files might have shown something like this:

In due course, I'll do a separate PR to tidy these and likely others.

[ca-d]

Sorry for not re-approving that sooner, LGTM.