Change Mypaint brush preset for krita5 (smaller list/better names/better thumbnails)

Mypaint brush preset selection for Krita 5.0

Situation before this MR: Screenshot of Krita 5 (Git Master): Mypaint presets don't feel part of the default presets

General purpose:
Many of the Mypaint presets feels like duplicate of Krita default brush preset (Krita4bundle). This MR will propose a smaller but more optimal set for Mypaint presets (located in plugins/paintops/mypaint/brushes/). Presets will also be decorated with thumbnails that feels part of Krita default.

1. Evaluation/Selection

Mypaint's engine strenght is into:

• Mixing/Dulling/Smudging engine: blender, wet knifes and rounded mix&paint brushes.
• Inking/Drawing with built-in stabilizer (distance smoothing).
• Experimental brushes.

2. Thumbnail

Brushes based on the Mypaint engine probably will benefit a (subtle) way to be diferentiated from the other:

a. Icon/Badge/Emblem

A Mypaint mini icon could be added in a top-left corner: a little canvas on a easel. It will help at Krita5 launch to discover the new feature and play with the brushes.

b. Color of the stroke

I plan no changes here: normal stroke use a dark gray color, mixing use violet, green for textured strokes, full blending use white... Krita already has a robust language with colors. I'll just respect that without making a Mypaint-themed color of stroke.

c. The tool 'sprite'

The representation of the tool in diagonal upper-right might be a good opportunity to use the color palette of Mypaint icon: sienna/camel color + a sky blue color. (eg. a tool preset for inking could be in this tones, the wood of the body part of a brush in this tones, etc...) That sounds subtle enough to underline the origin of the brush while being totally integrated into Krita's default list.

3. Naming

I plan to use the alphabetical sorting of the Krita list and distribute the brush presets into the list of default and according to their workflows: ink with ink, mix with mix, etc...