Minor improvements to About Xfce dialog

I was looking at the about system dialog in gnome and comparing it with the recently implemented system tab in the about xfce dialog and wanted to suggest 2 changes.

1. For the 'OS Type' line, it would be better to use the more easily understood '64-bit' rather than 'x86_64'.
2. Addition of a 'Graphics' line.

Looking at @ochosi's patch, I do see 'graphics' label mentioned, so I presume it is something that is intended to be implemented. af21b9a8