Provide a Convenience API for usage of Ajax Channels

The Usage of Wicket Ajax Channels is currently rarely documented and pretty low-level.
You have to build the Name of the Channel with a certain Format (name|d for DropChannels, name|s for Stackable Channels).
I don't know why this is so, but this feature is pretty cool, and I think we should provide a little convenience api for the usage of Channels.

I attached a patch that shows the idea - its pretty simple, lets just not use a String but an Objects that encapsulates the behavior with a little
Javadoc, so more people may find this feature.

WIth the Patch, all works as before, but one could provide an AjaxChannel for its Behavior and must not fiddle with Strings.

Thanks.