Save untitled files to the last active folder

[msujew]

What it does

Closes #13182

The changes to UserWorkingDirectoryProvider will now keep track of the latest opened navigatable resource. In case the current file doesn't use a valid URI scheme (i.e. untitled), it will now always default to the latest opened resource.

The changes in DefaultFileDialogService ensure that in case no resource has been opened yet, it will default to the first open workspace folder or the user home directory.

How to test

1. Create an untitled file.
2. Try to save it. The save dialog should point to the current workspace folder/home directory (in case of no workspace)
3. Open a different file in the workspace.
4. Try to save the untitled file again. The save dialog should point to the parent folder of the last opened file.
5. Running Save As... on existing files should behave as expected (i.e. save to their parent folder).

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[martin-fleck-at]

Change looks good to me and works as advertised, thank you @msujew!