consider making WidgetDetails member values private

[rmpowell77]

Currently:

template <typename ConcreteWidget, typename Underlying>
struct WidgetDetails {
...
public:
    std::optional<wxSizerFlags> flags;
    // these are common across the controls
    wxWindowID identity = wxID_ANY;
    wxPoint pos = wxDefaultPosition;
    wxSize size = wxDefaultSize;
    int64_t usingStyle {};
    Underlying** windowHandle {};
};

Consider making these private.

[rmpowell77]

More details:
https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-private
https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rh-protected

[rmpowell77]

Fixed in #74