Implement intuitive node creation and deletion at crossing points

It is now possible to interconnect a X crossing adding an additional wire ontop
starting from the crossing point, or removing the node by creating a wire and
spans over that point. T or L like crossing nodes can not be removed, as that
does not make any sense.
Fix double removal of wire from list, introduced in 5674c722, properly interconnect L-crossings with a node (not drawn)

The second fix is especially important for simulation, as only nodes can
interconnect wires. Just for the record, black dots guarantee
a connection between items, L-crossing connection dots are NOT drawn, as that
would just clutter the appearance. This was the main reason the issue
slipped through.
Add compare function to Coords

Keep in mind x-direction has priority over y-direction.
Extend save/load to allow nodes in the middle of wires

Allows storing and retrieving of x-crossings.
The wires are stored in chunks (ogo:points directives) to one <ogo:wire> directive
and are reloaded as single wires. These wires are then rejoined within
the `node_store_add_wire` logic.
TODO: Introduces a drawing glitch.