[Request] fix xfce versioning about inxi HOT 3 CLOSED

This is not a defect, but does appear to be an arch issue with package naming.

inxi uses this: xfdesktop --version

to get the xfce version, then falls back to the generic 4 if that app was not 
detected.

Check Arch to see if that package has a different name in Arch Linux. Or maybe 
it doesn't exist, we'll find out. I can use xfce4-panel if necessary but I 
prefer to use the same package names to avoid distro specific hacks in inxi, so 
let's find the actual standard arch name used for the desktop if possible. the 
panel can actually have a different version number than core xfce4 slightly, I 
just had that issue on a system.

This is waiting data from bug reporter, as noted above, until then will not be 
fixed.

xfce4-panel --version has been added as a fallback test, though I do not 
understand why it's needed since arch linux has this package:

https://www.archlinux.org/packages/extra/i686/xfdesktop/

but just in case I've added in 1.9.18 the second test.