[tuxpaint-stamps] package is too large and needs splitting into a default package and extras

This bug is splitted from #82510 and was first submitted as part of bug #155163. Currently tuxpaint-stamps-default package is ~155MB (215MB after installation) size!

From bug #155163:

"it would be nice to split very big (currently 155 MB) tuxpaint-stamps-default package at least into two - tuxpaint-stamps-default and tuxpaint-stamps-extra."

From bug #82510:

"2. There are lof of situations, when stamps are not needed, even unwanted, as sometimes existence of stamps limits creativity (kids keeps simply clicking on stamps instead of painting).
3. Even in description of tuxpaint package stamps are mentioned as optional, but in reality users are forced to install all stamps if they install tuxpaint :( From tuxpaint package description: "[..] Brushes and "rubber stamp" shapes can be dropped in and pulled out. For example, a teacher can drop in a collection of animal shapes and ask their students to draw an ecosystem."

Also package tuxpaint-stamps-default shares 21.3 MB of identical files with itself, see
https://dedup.debian.net/compare/tuxpaint-stamps-default/tuxpaint-stamps-default

Please remove the duplication by using symlinks, see bellow for more info how to to this automatically during build:
https://wiki.debian.org/dedup.debian.net#Tips_for_reducing_duplication_in_packages