RFC: thumbnail: Modify to follow the XDG Basedir Spec

[Vincent Untz]

Hi,

There's a patch in [1] to change the thumbnail spec to use the xdg
basedir spec to determine where to store thumbnails. Concretely, instead
of using ~/.thumbnails, $XDG_CACHE_HOME/thumbnails is used (falling back
to $HOME/.cache/thumbnails if $XDG_CACHE_HOME is not set).

I'm attaching the patch here for easier review.

Opinions?

Cheers,

Vincent

[1] https://bugs.freedesktop.org/show_bug.cgi?id=49607

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