Add thumbnail cache inspection tools

tools/clean-up-thumbnail-cache.sh

@@ -0,0 +1,15 @@
+#!/bin/sh -e
+# Remove thumbnails with URIs pointing to at this moment non-existing files.
+make pnginfo
+
+pnginfo=$(pwd)/pnginfo cache_home=${XDG_CACHE_HOME:-$HOME/.cache}
+for size in normal large x-large xx-large; do
+	cd "$cache_home/thumbnails/$size" 2>/dev/null || continue
+	find . -name '*.png' -print0 | PNGINFO_SKIP_TRAILING=1 xargs -0 "$pnginfo" \
+		| jq -r '.info.texts."Thumb::URI"' | grep '^file://' \
+		| grep -v '^file:///run/media/[^/]*/NIKON/' \
+		| perl -MURI -MURI::Escape -MDigest::MD5 -lne \
+			'print Digest::MD5->new()->add($_)->hexdigest . ".png"
+				if !stat(uri_unescape(URI->new($_)->path))' \
+		| xargs rm
+done