X backend clipboard properly negotiates data type with owner

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.

backends/x/window.go

@@ -305,6 +305,7 @@ func (window *window) Paste (callback func (data.Data, error), accept ...data.Mi
 
 	window.selectionRequest = window.newSelectionRequest (
 		selectionAtom, propertyAtom, callback, accept...)
+	if !window.selectionRequest.open() { window.selectionRequest = nil }
 	return
 }