minor fixes for trinity in get_wallpaper/set_wallpaper

[mike]

a small fix for trinity in get_wallpaper
*********************
# trinity
elif [ "$XDG_CURRENT_DESKTOP" == "Trinity" ]; then
#virtual desktop 1
        dcop kdesktop KBackgroundIface currentWallpaper 1 2> /dev/null
*************************

addition of a helpful comment in set_wallpaper
********************
# trinity
if [ "$XDG_CURRENT_DESKTOP" == "Trinity" ]; then
#display mode 4, 1-8
dcop kdesktop KBackgroundIface setWallpaper "$WP" 4 2> /dev/null
fi
**********************

[James Lu (jlu5)]

Merged with tweaks in https://github.com/varietywalls/variety/commit/23c5e476d5da5ecab659a9a80f60a7345a427e3f (I expanded the comments for a bit more clarity)