Check for thumbnails in the new location (Bug #11263)

The thumbnail managing standard was updated to change the location
where thumbnails are stored. This has been changed to
$XDG_CACHE_HOME/thumbnails/ unless $XDG_CACHE_HOME isn't defined,
at which point it's back to $HOME/.cache/thumbnails . This patch
has Thunar look in both places when the thumbnail is created and
reports the first one that's found.

Signed-off-by: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at>