Allow MTime and URI to be missing in shared thumbnails
Closes: #53 (closed)
This is a first jab at #53 (closed) -- 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.)