Try to combine consecutive window moves into one

Try to combine consecutive window moves into one

It often happens that we move region A to B and then we move a subset
of B to C. When possible we'd like to replace this with a move from
A directly to C, and a suplimentary move from A to the areas of B not
overwritten by C.

Getting an optimal move combiner seems quite complicated, but this
simple approach gets most of the interesting cases right and isn't
all to complicated.