portal_type_class: possible to overwrite methods or properties in mixin

Before this change, class hierarchy is like this when using mixin:

class MyClass(BaseClass, Mixin1....)

which is usually ok when classes don't override each other's

But if we want to overwrite BaseClass's method by using mixin to do more thing,

For example:

class BaseClass(object):
  def test(self):
    print 'base test'

class Mixin1(object):
  def test(self):
    super(Mixin,self).test()
    print 'mixin'

I want to display 'mixin base test' when call test, but it doesn't work

since priority of how methods are resolved is from left

to right: BaseClass----->Mixin1, it only display 'base test'

So the correct way to use mixin should be in reverse order:

class MyClass(Mixin1, BaseClass)