_HasDropDown: throws exception if widget doesn't have focus method

[bill]

_HasDropDown.js throws an exception if the subclass doesn't have a focus method. It calls this.defer("focus") unconditionally

The _HasDropDown.html test because the test widgets like "dd" have no focus method.

[bill]

In [30796]:

avoid exception when subclass has no focus method, fixes #16816 on trunk !strict

[bill]

In [30797]:

avoid exception when subclass has no focus method, fixes #16816 on 1.8 branch !strict