[Xmonad] darcs patch: clear out motion events when processing one motion event.

On my laptop anyhow (over six years old) this is completely necesary for adequate performance when something expensive is being done in the drag loop. And it's harmless, so far as I can tell. David Sun Aug 19 17:23:51 PDT 2007 David Roundy <droundy@darcs.net> * clear out motion events when processing one motion event. This is important if the hook is slow (e.g. try adding "float w" to the window-dragging hook), as it allows xmonad to keep up with the motion of the mouse.