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.

interface/wx/combo.h

@@ -751,6 +751,47 @@ public:
     */
     virtual void SetInsertionPointEnd();
 
+    /**
+        Uses the given window instead of the default text control as the main
+        window of the combo control.
+
+        By default, combo controls without @c wxCB_READONLY style create a
+        wxTextCtrl which shows the current value and allows to edit it. This
+        method allows to use some other window instead of this wxTextCtrl.
+
+        This method can be called after creating the combo fully, however in
+        this case a wxTextCtrl is unnecessarily created just to be immediately
+        destroyed when it's replaced by a custom window. If you wish to avoid
+        this, you can use the following approach, also shown in the combo
+        sample:
+
+        @code
+            // Create the combo control using its default ctor.
+            wxComboCtrl* combo = new wxComboCtrl();
+
+            // Create the custom main control using its default ctor too.
+            SomeWindow* main = new SomeWindow();
+
+            // Set the custom main control before creating the combo.
+            combo->SetMainControl(main);
+
+            // And only create it now: wxTextCtrl won't be unnecessarily
+            // created because the combo already has a main window.
+            combo->Create(panel, wxID_ANY, wxEmptyString);
+
+            // Finally create the main window itself, now that its parent was
+            // created.
+            main->Create(combo, ...);
+        @endcode
+
+        Note that when a custom main window is used, none of the methods of
+        this class inherited from wxTextEntry, such as SetValue(), Replace(),
+        SetInsertionPoint() etc do anything and simply return.
+
+        @since 3.1.6
+    */
+    void SetMainControl(wxWindow* win);
+
     //@{
     /**
         Attempts to set the control margins. When margins are given as wxPoint,