Improve validation of wxCheckBox styles.

Improve validation of wxCheckBox styles.

Detect when incompatible styles are used (this required changing the value of
wxCHK_2STATE to be non-null) and sanitize the styles (after asserting) in this
case.

Put the validation code in wxCheckBoxBase instead of having slightly different
versions of it in port-specific wxCheckBox implementations.

Add a unit test checking that the expected asserts are indeed generated.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65824 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775