Abort a drag when a keynav drop has not destination

Abort a drag when a keynav drop has not destination

This was claimed to cause problems for Chromium, see bug 599130.

Also work around apparent rounding errors in XIWarpDevice by
setting the 'small step' for keynav dnd to 2 instead of 1 - I notice
that a warp seems to sometimes warp a little less than I tell it to,
and if I tell it to only move by 1 pixel then moving less means
that you are stuck.