Rework triple click multi paste avoidance

This fixes a memory leak and, frankly, speculative cases where
double-click selected text that shouldn't be copied, is copied.
- don't leak QMimeData in case it isn't eventually submitted to
  the clipboard - don't even create it
- get rid of the separate flag for pending double-click copy text,
  just check if the paste text is empty
- Consolidate the two QTimers started after a double click. QTimer
  has some guarantees about order of starting and order of
  triggering of timers, but it's more obvious to just do things
  from one timer. Also slightly more efficient.
- Always cancel double-click paste if any other paste happens
  before the timeout - add code to that effect in the most generic
  copying code paths. I'm not quite sure if all possible cases
  work correctly now. Many things can happen with all the
  selection modes supported + all the possible user input events.