This MSW-specific hack is actually needed by all windows containing more than
one control, even if they don't derive from wxPanel (which is just the most
commonly used class for such windows), otherwise the parts of the window not
covered by the child controls won't have the correct appearance when the
window itself is inside a wxNotebook.
So do this for all classes inheriting from wxNavigationEnabled<>, notably this
fixes the wrong background for all kinds of picker controls (wxDirPickerCtrl,
wxFilePickerCtrl, ...) when they're used inside a wxNotebook.
After moving this method out of wxPanel, src/msw/panel.cpp became empty, so
also delete it and remove it from {bake,make,project} files.
