Add support for generating PNGs in the desired size to the build-pngs.sh script

This commit adds an optional argument that may be passed to build-pngs.sh
which will cause PNGs of the specified size to be generated.  Default
size generated remains at 16x16
Minor tweak to always include size in PNG folder path
Revise the makefile and create-all-folder scripts to create and copy PNGs in various sizes

This commit heavily refactors the create-all-folder script to use functions to reduce
verbosity and make it easier to add new folders in the future.  It also copies the PNGs
in all the available sizes to the all folder.

The build-pngs target in the Makefile will now generate PNGs in 16, 32, 64 and 128 pixel
sizes