The clipboard API has been changed to allow an application to
accept a number of different mime types, and the X backend will now
check the accepted types list against the owner's TARGETS list and
choose the best one.
In X, clipboard/selection data is specific to each window, and it
may take some time before the clipboard data is fully transferred.
This actually makes sense because there can be entire images in
the clipboard and it is important the clipboard API supports large
file transfer. Because of this, the Copy and Paste methods have
been moved into Window, and Paste now returns a channel.
