gimp_brush_editor_update_brush(): only set the brush property whose
GtkAdjustment has changed, and only set it if it is different from the
current value. Before, simply setting a brush on the editor would
cause all properties to be written back to the brush unconditionally.

gimp_brush_editor_notify_brush(): unrelated cleanup: use brush
accessors instead of direct struct access, remove an update of the
radius property from the shape updating code that was probably left
over from copy and paste.