Be relocatable on Linux.

Instead of relying on build-time POEDIT_PREFIX value to locate the data
at runtime, use runtime-detected location on Linux.  Still falls back to
POEDIT_PREFIX on other Unix platforms.
Non-ugly welcome screen buttons under GTK+.
Use XDG_CONFIG_HOME location on Unix.

Use the now-standard ~/.config location for the configuration. Migrate settings from the previous location if it doesn't exist yet.