message: python: allow to customize Python namespaces
  extra methods through PYOPENCV_EXTRA_METHODS_<NS_UPPER> macros
  extra constants through PYOPENCV_EXTRA_CONSTANTS_<NS_UPPER> macros
diff: @@ -976,6 +976,8 @@ class PythonWrapperGenerator(object):
  if func.isconstructor:
  continue
  self.code_ns_reg.write(func.get_tab_entry())
  + custom_entries_macro = 'PYOPENCV_EXTRA_METHODS_{}'.format(wname.upper())
  + self.code_ns_reg.write('#ifdef {}\n {}\n#endif\n'.format(custom_entries_macro, custom_entries_macro))
  self.code_ns_reg.write(' {NULL, NULL}\n};\n\n')
  self.code_ns_reg.write('static ConstDef consts_%s[] = {\n'%wname)
  @@ -984,6 +986,8 @@ class PythonWrapperGenerator(object):
  compat_name = re.sub(r"([a-z])([A-Z])", r"\1_\2", name).upper()
  if name != compat_name:
  self.code_ns_reg.write(' {"%s", static_cast<long>(%s)},\n'%(compat_name, cname))
  + custom_entries_macro = 'PYOPENCV_EXTRA_CONSTANTS_{}'.format(wname.upper())
  + self.code_ns_reg.write('#ifdef {}\n {}\n#endif\n'.format(custom_entries_macro, custom_entries_macro))
  self.code_ns_reg.write(' {NULL, 0}\n};\n\n')
  def gen_enum_reg(self, enum_name):
  