1. renamed wxFontMapper::IsWxFontMapper() to IsDummy() (with reverse semantics)

1. renamed wxFontMapper::IsWxFontMapper() to IsDummy() (with reverse semantics)
2. added wxFontMapper::Reset() to only do the cast needed when deleting
   the font mapper object once
3. reset the dummy font mapper created during the app initialization in
   wxFontMapperModule and not in init.cpp

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@39624 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775