xmonad_log_applet: add packages

This can be used in tandem with XMonad for nice xmobar-like labels in Xfce and Gnome{2,3}. I haven't tested it in gnome2 and was desperate to find where `libpanelapplet-4.0` from gnome3 resides, but on Xfce this works and is very useful.
What about renaming all that gnome2Support ecc. combinatorial explosion to `panelFlavor` to be one of "gnome2", "gnome3" or "xfce4". Then assert `panelFlavor == "gnome2" || ...`.