In wxGTK2 the wxMessageDialog is not a real wxDialog so its m_widget

In wxGTK2 the wxMessageDialog is not a real wxDialog so its m_widget
will always be NULL there will be a wxCHECK asserts triggered whenever
calling base class methods that are valid calls (or just ignored) on
other wx ports.  Added a set of macros to be used to test m_widget
that will bypass the wxFAIL if the window is a wxMessageDialog.  This
will silence those asserts if the window is a wxMessageDialog and will
let the method doing the check just be ignored in that case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@27041 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775