Enable the delete button that's been hidden away so far. When clicked, we
suffix the cached json file with '.deleted'. This also causes a popup with an
undo button to appear (taken from Nautilus). When clicked, that button will
restore the drawing again.
This is really all just renaming anyway, because nothing ever gets deleted
here.
An extra filter is needed to skip Tuhi drawings that have been deleted in the
GUI - we don't want to add the drawings we just deleted again just because
Tuhi still keeps them in cache.
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