[Direct Edit] Fix Qt slot signature preventing to edit custom xpath

Problem

The consequent improvement done in 4.5.1 with NXDRIVE-1768 introduced a bug in few Qt slots arguments.

Analysis

Drive is using a Qt signal to launch Direct Edit on a given document.

The function recieving the signal has that signature:

@pyqtSlot(str, str, str, str)
def edit(self, server_url: str, doc_id: str, /, *, user: str = None, download_url: str = None) -> None:
    # ...

In recent Python versions (3.8+), / means that all arguments on its left are "positional-only", e.g: one must call the function like edit(arg1, arg2, ...) and edit(server_url=arg1, doc_id=arg2, ...) is forbidden.
* means that all arguments at is right are "keyword-only", e.g.: one must pass a key=value pair for those arguments such as user="alice" and download_url="...".

Combined, that means we forced the function to be called like:

edit("https://...", "123-456-789-0", user="alice", download_url="nxfile/...")

But the Qt signal does not understand that new notation: it recieved 4 arguments and does not care if one is "position-only" or "keyword-only".
Results: user and download_url were always set to None.

That's the root cause of the problem.

Then, Drive was seeing an empty download_url and so it was using the default blob location, which is file:content (download_url is used to guess an eventual custom xpath).
But that xpath was not found as blobs are effectively stored elsewhere.

Verification

That command mimics a click on the Direct Edit button in from Web-UI:

$ python -m nxdrive "nxdrive://edit/http/192.168.2.39:8080/nuxeo/user/ndt-robert-67422/repo/default/nxdocid/152fc209-f946-4d3b-b128-485d21172be0/filename/attachment.txt/downloadUrl/nxfile/default/152fc209-f946-4d3b-b128-485d21172be0/files:files/0/file/attachment.txt"

In 4.5.1, Drive will recieve those arguments:

Direct Editing doc_id='152fc209-f946-4d3b-b128-485d21172be0' on server_url='http://192.168.2.39:8080/nuxeo' for user=None with download_url=None

In 4.5.0 and upcoming 5.0.0:

Direct Editing doc_id='152fc209-f946-4d3b-b128-485d21172be0' on server_url='http://192.168.2.39:8080/nuxeo' for user='ndt-robert-67422' with download_url='nxfile/default/152fc209-f946-4d3b-b128-485d21172be0/files:files/0/file/attachment.txt'

—

Edit: a similar issue was present in EngineModel.get().