Prevents focus change due to mouse jitter

Yet another request...

This causes the `focus_follows_pointer` logic to ignore small mouse
jitters. This means that accidentally touching a laptop's trackpad
doesn't cause the focus to switch.

This patch prevents the pointer from causing a focus switch if the
pointer is moved fewer than 10 "Manhattan distance" (`dx+dy`) pixels
within a second.

I can make it configurable if you want but 10px in 1s is a pretty reasonable default.