umbrello/umlviewimageexportermodel.{h,cpp} (s_supportedMimeTypesList,...

umbrello/umlviewimageexportermodel.{h,cpp} (s_supportedMimeTypesList, s_supportedImageTypesList) : Change to pointer.

umbrello/umlviewimageexportermodel.cpp
- In function supportedImageTypes, if s_supportedImageTypesList is null
  then allocate new QStringList to it.
- In function supportedMimeTypes, if s_supportedMimeTypesList is null
  then allocate new QStringList to it.
- These changes help avoid "C++ static initialization order fiasco", see
  https://en.cppreference.com/w/cpp/language/siof