Tk Additions

These functions are Windows-specific. They issue an error under a different platform.

tk2dde(topic = NULL)
tk2dde.services(service = "", topic = "")
tk2dde.exec(service, topic, command, async = FALSE)
tk2dde.poke(service, topic, item, data)
tk2dde.request(service, topic, item, binary = FALSE)

`topic`	the 'topic' to reach or expose. A DDE server is accessed as 'service'\|'topic'. In the case of `tk2DDE()`, a non null topic activates the DDE server, and a null topic deactivate it.
`service`	the name of the service to reach. In `tk2DDEservices`, if both service and topic are empty, the list of all available DDE service is returned, otherwise, only available topics for a given service are listed.
`command`	a string with the command to run in the external application (syntax depends on the server).
`async`	is a command run asynchroneously (returns immediately, before the command is processed), or not?
`item`	the concerned item (usually a variable name, a range in a worksheet, etc...).
`data`	the new value for the item.
`binary`	should the return bez treated as binary data or not?