#56 Platform-specific config,data,cache location

[ratijas]

On Linux platform, consider moving Golly configuration, data and cache files into user specific directories as specified by FreeDesktop "XDG Base Directory Specification".

• $XDG_CONFIG_HOME for config (defaults to ~/.config)
• $XDG_DATA_HOME for other user specific data (defaults to ~/.local/share)
• $XDG_CACHE_HOME for removable cache (defaults to ~/.cache)

It helps to keep home directory clean, and also to simplify backup configurations (e.g. to avoid backing up cached data).