fix #63: window jumping to origin position when dragging
fix #63: window jumping to origin position when dragging

We need to make sure that the StackSet contains the right
position for the window at all times while dragging. Previously,
the window was only placed at a different position. If, for any
reason, a layout refresh happens while the window is being dragged,
then the window will jump back to the old position because the StackSet
still contains the position of the window where it was as the drag started.

This patch fixes that issue by calling float after each mouse movement, which
ensures that the position for the window is updated in the StackSet.

Fixes #63

xmonad/xmonad