Add current working directory info to the custom action edit dialog and possibly a cwd substitution also

Filing this against thunar 4.20.2 on Xubuntu 25.04, for the custom action edit dialog, ie this one:

I suggest that an item be added to the info tips, that the current working directory of the launched action will be the one currently being viewed, if this is actually correct, of course (based on trial and error - I think it is).

I also suggest to add an actual substitution, eg %c, for the current directory, regardless of what is selected.

I actually think that the open-terminal action should default to the viewed directory, not any selected one, but that's subjective and means changing defaults. However, there should be a way to configure it and it should be cleanly documented.