Nets sometimes don't undo move properly

[Siddharth Shekar]

1. Create a horizontal net
2. Create a vertical net such that it passes through one of the endpoints of the net drawn in 1 (the vertical net should now be split into 2 and a dot should be added).
3. Select all the items and move them to a new location.
4. Perform the undo operation.

Expected behaviour: All the items come back to their position before the move. Observed behaviour: One of the nets does not move back while the other 2 and the dot do.

[Siddharth Shekar]

Fixes issue #27 by remembering the location where the item was when it was deleted. Fixed some more crashes relating to net splitting/merging.

→ <<cset 32bbe488b54e>>