Improved ability to append and update existing chat, fix to groupInterval, fix to Copy

I've added some new functionality to allow appending of new chat objects to a live table. It uses the snap interval, but does not make any attempt to group cells together (I thought that was an appropriate choice, but then I also don't use the grouping feature)

It is also now possible to use the indexPath entry of a ChatData object to reload a subset of the table. This is primarily useful if the status changes, because the view is currently a readonly property. 

The copy menu now checks the data type before it is shown, and will only display for supported chat types (label, image, imageview).

GroupInterval does a >= comparison so that a value of 0 can be used to disable grouping.

The default background of a chat when a bubble image is not supplied is clear. This was necessary for situations when no bubble is desired. The alternative would be to create a transparent image, but that seems like a roundabout way to solve the problem.