Allow MTime and URI to be missing in shared thumbnails

Open chrysn requested to merge chrysn/xdg-specs:thumb-metadata-tolerance into master

Closes: #53

This is a first jab at #53 -- may work as is already, but is intended to give a bit of more substance and momentum to the (so far onesided) discussion.

(For lack of concentration that was previously chrysn/xdg-specs!1 (closed), which this MR is now taking over for.)

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