Now the masking brush changes according to these requirements:

1) When the brush size is changed in the brush editor, masking
   brush size is kept unchanged

2) When the brush size is changed using quick-controls, that is
   toolbox slider, shift+gesture, HUD display, the masking brush size
   is changed proportionally.

3) Technically, the masking brush supports disabling this "dependent
   size" functionality, but I'm not sure if it should be visible in GUI.
   It is already overcomplicated.

Technical changes:

1) Now the brushes are always **copied** when fetched from the
   brushes registry. That is, if you load the brush using
   KisBrush::fromXML(), you will always have your own copy of the
   brush object, not shared with the one in the server.

2) For the efficiency reasons, the brush tip QImage will be lazily
   shared with the one on the server using the Qt's internal algorithm.
   If you change the brush tip in you copy of the brush, Qt ...