Support for proper renaming of XDG user DIRS

Currently, renaming of a XDG User DIR (like e.g. ~/Downloads) followed by reboot will lead to having no special icon for that folder anymore (among other possible downsides).

The idea is, to either directly update ~/.config/user-dirs.dirs or use some library to do so, in order to simplify proper renaming of these folders.

Version information

thunar master

Steps to reproduce

1. mv ~/Downloads ~/Downloads.new
2. open thunar
3. No special icon will be used anymore for the "Downloads.new" folder

Additional information

I suppose we could check if the folder to rename is a XDG-user-dir, and if so, call some xdg-user-dirs-update method (or at least show some warning before showing the thunar rename dialog).

According to this askubuntu answer, nautilus is/was able to update the xdg dirs properly.