wx public headers are not supposed to include platform-specific headers
defining many macros that can conflict with the identifiers defined in
the application code. In this particular case, including CoreServices.h
ultimately #included AssertMacros.h, which by default on older SDKs
(<10.12) introduces various macros whose names very easily conflict with
user code.
For example, if you #included <wx/fswatcher.h> in your own code, and
your code happened to contain a symbol called 'check', or 'verify',
compilation failed.
Fix this by using pImpl idiom to move the variable requiring a type
defined in the SDK header into the source file.
Closes https://github.com/wxWidgets/wxWidgets/pull/1666
