Support default internet radio stations (bgo#548197)

At first start, read DATADIR/stations/*.xspf and import them into the user DB
in the InternetRadio extension. This allows distros or admins to provide
some default radio stations; since we support multiple ones, these can
even be locale specific when language packs or region specific packages
install them.

We only look once for those xspf files, on the first start, and never
again, even if there were no files found or errors while loading the
files.

Signed-off-by: Bertrand Lorentz <bertrand.lorentz@gmail.com>