Use different members for storing wxBitmap and wxIcon instead of using a
single pointer for storing a heap-allocated copy of either of them
because this is simpler, more efficient due to avoiding unnecessary
conversions and RTTI, and will be simpler to generalize to using
wxBitmapBundle instead of wxBitmap in the upcoming commits.
As a side effect, don't assert in GetIcon() or GetBitmap() if there is
no valid icon or bitmap, as this is inconsistent with the other ports
and wasn't even consistent between these functions in wxMSW itself.
There should be no other changes in behaviour.
c1abbe6473