Support XDG Base Directory Specification (LP: #1074448).
Onboard's data directory moves from ~/.onboard to the XDG data directory, which will usually be ~/.local/share/onboard. Existing data in either ~/.onboard or ~/.sok is migrated to the new location.
System defaults are looked up in XDG_CONFIG_HOME, XDG_CONFIG_DIRS and the previous locations, in this order.