Add new app icon and switch to rDNS icon name

So here goes the final iteration (hope you like it) of the new Thunar icon.

We currently didn't provide any 64px icons for any Xfce applications in the first run, so I hope you're ok with losing that size (and 24px, but that one probably matters less) for now. We may add those later to each app.

I also dropped the pixmaps folder with the old about logo, which may have been "borrowed" from some school.

Caveat: I only replaced all the instances of the thunar icon I could find in the code, please double-check if I didn't overlook anything...