Allow using custom main window in wxComboCtrl

Add wxComboCtrl::SetMainControl() which can be used to use some other
window instead of the default wxTextCtrl as the main window of the
combo control.

Update the sample and the documentation to show the new function.

include/wx/combo.h

@@ -197,6 +197,10 @@ public:
     // get the popup window containing the popup control
     wxWindow *GetPopupWindow() const { return m_winPopup; }
 
+    // Set the control to use instead of the default text control for the main
+    // (always visible) part of the combobox.
+    void SetMainControl(wxWindow* win);
+
     // Get the text control which is part of the combobox.
     wxTextCtrl *GetTextCtrl() const { return m_text; }
 
@@ -623,8 +627,12 @@ protected:
     // This is used when control is unfocused and m_valueString is empty
     wxString                m_hintText;
 
-    // the text control and button we show all the time
+    // This pointer is non-null if we use a text control, and not some other
+    // window, as the main control.
     wxTextCtrl*             m_text;
+
+    // the window and button we show all the time
+    wxWindow*               m_mainWindow;
     wxWindow*               m_btn;
 
     // wxPopupWindow or similar containing the window managed by the interface.