Template export: test image dimensions & file CRC

Final couple of tests to let us avoid creating a template that's
identical to one that already exists.  All told, we test the
following (and the first thing that matches causes us to stop
testing that template, and move on to the next, as it appears
to not be identical to the saved image, in its current state)

 + filename prefix (based on saved image's filename)
 + file size
 + image dimensions
 + image contents (via CRC)

docs/CHANGES.txt

@@ -43,9 +43,9 @@ https://tuxpaint.org/
       + If the original image has already been exported as a template,
         it will not be re-saved.  We check, in this order:
         - identical filename prefix (based on original saved picture)
-        - identical file sizes
-        - TODO identical image dimensions
-        - TODO identical image data
+        - identical file sizes (bytes)
+        - identical image dimensions (pixels)
+        - identical image data (CRC checksum)
       Closes https://sourceforge.net/p/tuxpaint/feature-requests/236/
       Bill Kendrick <bill@newbreedsoftware.com>
       + TODO - Need to allow the ability to turn this feature off.