Depot: prevent Ox from dumping goods when changing instructions

[eugenikus8]

• Changing Source or Destination without losing resources.
• If Destination is destroyed or refuses to accept goods, carts will automatically return to Source.
• If Source and Destination is destroyed, carts will wait for new instructions.
• Changing transported resource type is not possible while carts are still in city.

Ways to change the type of transported resource:

• Delete Cart Depot.
• Temporarily disable Cart Depot (all carts will instantly disappear).
• Click the "Recall" button in Cart Depot menu - carts will return to Source to unload.
• Select another Destination for unloading, then press "Recall" button. To prevent carts that have returned to depot from going back to source for goods, you can use the new "Clear source" button - then carts will not spawn and will wait for new instructions.

Added depot cart pushers status

[crudelios]

Apart from a few code formatting suggestions, code looks good.

I haven't tested for functionality or bugs though.