Register command as a handler to supply the X selection owned by this widget when another application requests it. When the selection is retrieved, command is called with two arguments, the starting character offset and the maximum number of characters to return, and must return at most that many characters of the selection starting at that offset; for very long selections it is called repeatedly with increasing offsets. The keyword argument selection names the selection (default PRIMARY) and the keyword argument type gives the form of the selection that the handler supplies (such as 'STRING' or 'FILE_NAME', default STRING).