Add wxListBox::GetTopItem() and GetCountPerPage()

Implement these methods for all the major ports, add them to the widgets
sample and documentation.

Closes #17189.

src/common/lboxcmn.cpp

@@ -33,6 +33,15 @@
     #include "wx/log.h"
 #endif
 
+// the spacing between the lines (in report mode)
+static const int LINE_SPACING = 0;
+
+#ifdef __WXGTK__
+static const int EXTRA_HEIGHT = 6;
+#else
+static const int EXTRA_HEIGHT = 4;
+#endif
+
 extern WXDLLEXPORT_DATA(const char) wxListBoxNameStr[] = "listBox";
 
 // ============================================================================
@@ -340,4 +349,23 @@ void wxListBoxBase::EnsureVisible(int WXUNUSED(n))
     // call SetFirstItem() but this is probably even more stupid)
 }
 
+wxCoord wxListBoxBase::GetLineHeight() const
+{
+    wxListBoxBase *self = wxConstCast(this, wxListBoxBase);
+
+    wxClientDC dc( self );
+    dc.SetFont( GetFont() );
+
+    wxCoord y;
+    dc.GetTextExtent(wxT("H"), NULL, &y);
+
+    y += EXTRA_HEIGHT;
+
+    return y + LINE_SPACING;
+}
+
+int wxListBoxBase::GetCountPerPage() const
+{
+    return GetClientSize().y / GetLineHeight();
+}
 #endif // wxUSE_LISTBOX