Add wxListBox::GetTopItem() and GetCountPerPage()

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

Closes #17189.

include/wx/gtk/listbox.h

@@ -74,6 +74,8 @@ public:
 
     virtual void EnsureVisible(int n) wxOVERRIDE;
 
+    virtual int GetTopItem() const wxOVERRIDE;
+
     virtual void Update() wxOVERRIDE;
 
     static wxVisualAttributes

include/wx/listbox.h

@@ -72,6 +72,9 @@ public:
     // necessary
     virtual void EnsureVisible(int n);
 
+    virtual int GetTopItem() const { return wxNOT_FOUND; }
+    virtual int GetCountPerPage() const;
+
     // a combination of Append() and EnsureVisible(): appends the item to the
     // listbox and ensures that it is visible i.e. not scrolled out of view
     void AppendAndEnsureVisible(const wxString& s);
@@ -133,6 +136,8 @@ protected:
     // single selection mode on platforms other than MSW).
     void UpdateOldSelections();
 
+    wxCoord GetLineHeight() const;
+
 private:
     wxDECLARE_NO_COPY_CLASS(wxListBoxBase);
 };

include/wx/msw/listbox.h

@@ -95,6 +95,9 @@ public:
 
     virtual void EnsureVisible(int n);
 
+    virtual int GetTopItem() const;
+    virtual int GetCountPerPage() const;
+
     // ownerdrawn wxListBox and wxCheckListBox support
 #if wxUSE_OWNER_DRAWN
     // override base class virtuals

include/wx/osx/core/private.h

@@ -609,6 +609,7 @@ public:
     // display
 
     virtual void            ListScrollTo( unsigned int n ) = 0;
+    virtual int             ListGetTopItem() const = 0;
     virtual void            UpdateLine( unsigned int n, wxListWidgetColumn* col = NULL ) = 0;
     virtual void            UpdateLineToEnd( unsigned int n) = 0;
 

include/wx/osx/listbox.h

@@ -108,6 +108,8 @@ public:
 
     virtual void EnsureVisible(int n) wxOVERRIDE;
 
+    virtual int GetTopItem() const wxOVERRIDE;
+
     virtual wxVisualAttributes GetDefaultAttributes() const wxOVERRIDE
     {
         return GetClassDefaultAttributes(GetWindowVariant());