Add option to permanently hide warning infobar when running Thunar on root account

Version information

• Thunar 4.18
• Slackware-build Puppy Linux

Steps to reproduce

• Run thunar as root. Puppy Linux uses root account by default.

Current behavior

• Infobar warning about thunar on root account always shown. Unable to dismiss the message permanently, it always shows when running thunar on root. The current workaround was resorting to gtk.css to hide that infobar warning. Adding an option to root account hide warning infobar permanently in thunar settings or in xfconf entry will be the best choice instead to control that infobar message instead of gtk.css hacks