Finalize preset_icons folder and add new emblems and thumbnails

This commit gather a general work over the preset_icons folder.
I added new thumbnails, repainted some of them for them to look
like part of the same family. All new png were optimized with
pngquant[1] and pngcrush[2] to save a lot of size for the future
appimage, packages and installer (at the scale of Krita large
distribution, I like to think this optimisation saves a lot of
energy).

I also deleted duplicate thumbnails finishing by _a.png.

[1] pngquant --quality=75-85 --force --ext ".png" target.png
[2] pngcrush -rem allb -brute -reduce target.png output.png