Add wxAddRemoveCtrl class.

This is a simple high level helper combining an arbitrary control showing
multiple items with the buttons allowing to add items to and remove items from
this control, but using the buttons and the layout appropriate for the current
platform.

Add the implementation itself, an example of using it to the dialogs sample
and the documentation.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78462 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -66,6 +66,7 @@ All (GUI):
 
 - Allow requesting modern (3.x+) OpenGL version in wxGLCanvas (Fabio Arnold).
 - Allow customizing window shown by wxBusyInfo.
+- Add wxAddRemoveCtrl.
 - Add wxEVT_MAGNIFY mouse event (Joost Nieuwenhuijse).
 - Make results of wxDC::DrawEllipticArc() consistent across all platforms.
 - XRC handler for wxAuiToolBar added (Kinaou Hervé, David Hart).