Changed parameter Scale to Size for predefined brushes

It has a few benefits:

1) It is easier for painters to operate with size than with scale
2) There is no cross-rounding problems between Scale and Size sliders.
   The existence of the Scale slider caused the values of the Size slider
   to be inexact.
3) Now there is a special 'Reset Brush' button that resets the scale of
   the brush to 1.0 and reloads other values.

BUG:322648,318940