* Use "$XDG_DATA_HOME/calcurse" for data files
* Use "$XDG_CONFIG_HOME/calcurse" for config files
* "$XDG_DATA_HOME" defaults to "$HOME/.local/share"
* "$XDG_CONFIG_HOME" defaults to "$HOME/.config"
* If "$HOME/.calcurse" exists, then it will be used instead for backward
compatibility.
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>