Template/Starter options: Ability to specify gravity

e.g. if the aspect ratio of the template/starter is wider than
that of the canvas, and "autoscale=vertical" (or "=both")
is specified, before it would chop the top and bottom of the
image off, displaying the vertical center of it on the canvas.
Now you can ask it to gravitate towards the top or bottom, if they
have content that's of higher significance than the opposite side.

h/t Pere & Albert for the suggestion.

TODO - Document, set some suitable defaults for images we ship.

docs/CHANGES.txt

@@ -7,7 +7,7 @@ Various contributors (see below, and AUTHORS.txt)
 https://tuxpaint.org/
 
 
-2023.February.10 (0.9.29)
+2023.February.12 (0.9.29)
   * Improvements to "Stamp" tool:
     -----------------------------
     * Stamps may now be rotated.
@@ -74,9 +74,13 @@ https://tuxpaint.org/
       the image's top/bottom and/or left/right, and if not both,
       whether to smear the sides of the starter/template to the
       edges of the canvas (the prior behavior, and the default),
-      or use a specific solid color background.
+      or use a specific solid color background.  When scaling,
+      gravity may be specified, to ensure that a particular side or
+      corner of the starter or template is included.
       Closes https://sourceforge.net/p/tuxpaint/feature-requests/190/
       Bill Kendrick <bill@newbreedsoftware.com>
+      with suggestions from Albert Cahalan <acahalan@gmail.com>
+      and Pere Pujal i Carabantes <perepujal@gmail.com>
 
       TODO:
        + Document