Support multiple flavors

[alecazam]

There is no way to specify that an item has multiple flavors/representations on the clipboard. mozGetDataAt and mozSetDataAt got this correct with an index that was in common across the flavors of a given item. This is also reflective of the native clipboard API. Adding a series of flavors to the clipboard as individual items of a DataTransfer object doesn't help the receiver determine if they're a flavor or unique item.

items: foo.png foo.jpg <- single item, two flavors
items: foo.png bar.jpg <- multiple items, two unique items
items: foo.png foo.jpg, bar.png bar.jpg <- multiple items, two flavors of the each item

In the DataTransfer item list, the index needs to be specified since the item increments
item[0] item[1]
index: 0 index: 0
name: foo.png name: foo.jpg
blob/file blob/file