Fix too big masking brushes created in the brush editor (and causing crashes)

This merge request fixes too big brushes generated by the masking brush feature. Now all the old-time todo items are completed:

1. The brush for the brush editor has properly limited size (that was the reason of the crashes)
2. When the brush size is clipped, the user sees a warning in the brush editor
3. This warning disappears only when the user changes the masking brush. Changing settings of the normal brush don't reset this warning, the brush still has a correct proportional linking with the main brush.
4. The brush size slider in the masking brush editor has correct range (0...3000 px by default)

BUG:440938