Since OS X 10.12 it has been named macOS so it makes sense
to reference it in documentation as such, even when it
sometimes refers to older versions which were called (Mac) OS X.
First of all, do define it under non-MSW platforms.
Second, don't crash in it when running under XP where wxTaskBarButton is not
available.
Also add IsAvailable() method to check for its availability explicitly and add
a demonstration of this class to the dialogs sample.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77706 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
