FilePicker: add ability to specify tags

[madbob]

Related to the use case of the "Save As" mode described in #2491

Add ability to specify tags while saving a file, eventually with an option to pre-populate the list of assigned tags.

[susnux]

If you save a file the file is not yet existing so the filepicker could not assign any tags.
So this is already possible with some custom logic, like you could provide a custom button "assign tags" and offer a tag selector dialog.
Then save this locally in the app state until the picker resolved.
Then you save the file and apply the tags from your app.