better handling for icons/sounds placement

(moved from issue #32)

Let's say that app icon and sounds should be placed in the ~/.local/share/quakelaunch dir (or $XDG_DATA_HOME/quakelaunch) Maybe even add explicit steps for that, or a helper script to install them, and then default to using the ~/.local/share/quakelaunch path in the .desktop file and in the example sound paths in the default config.