Change hwloc-ls.desktop in lstopo.desktop and only install if graphical

hwloc-ls doesn't always point to a graphical lstopo, while hwloc-ls.desktop
wants a graphical application. Use lstopo instead (the change is invisible
from the menu) and don't install it unless lstopo has Cairo support.

By the way, fix some issues in the desktop file (Categories Application
is deprecated, comment should be Comment).
Thanks to Jirka Hladky for reporting these issues.