Fix display of empty wxStaticBoxSizers.

wxStaticBoxSizer should be considered shown if its static box is shown, even
if it has no elements (or all of them are hidden).

Closes #14698.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73729 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/sizer.h

@@ -715,6 +715,10 @@ public:
 
     void Show(bool show) { ShowItems(show); }
 
+    // This is the ShowItems() counterpart and returns true if any of the sizer
+    // items are shown.
+    virtual bool AreAnyItemsShown() const;
+
 protected:
     wxSize              m_size;
     wxSize              m_minSize;
@@ -1033,6 +1037,7 @@ public:
 
     // override to hide/show the static box as well
     virtual void ShowItems (bool show);
+    virtual bool AreAnyItemsShown() const;
 
     virtual bool Detach( wxWindow *window );
     virtual bool Detach( wxSizer *sizer ) { return wxBoxSizer::Detach(sizer); }