[FEATURE] Implement MyPaint-style brush outline

This patch does two major things:

1) Implements MyPaint-style brush outline, which is simply a circle.
   It is needed when the brush tip is too complex and people do not want
   to be distracted by the details of it.

2) Configuration GUI of the brush cursor is changed. Now you can choose
   separately, which cursor and which outline you want to see in Krita.
   There is no that huge list now! :)

CC:kimageshop@kde.org