To start a clipboard incr transfer the client has to delete the INCR
window property as the reply to the selection. This deletion generates a
property change event with the type delete, errornously triggering the
incr processing of the data. This patch corrects this by ignoring
property deletions.
Due to the logic in the event loop property events may get filtered out
that were clipboard related. This changes ensures the clipboard event
handler code gets to run first avoiding this issue.
