Logistic groups

Some refinement to the Logistic monorails. Slightly cleaner UIs:

The Group field separates towers into logistic groups or networks. If a group name is set, towers only exchange cars within their group, even if they share a rail network with other groups. This replaces the old overflow mechanism which was a bit inflexible.

Towers with a higher Priority field receive item shipments first.

The old Send Car button on export towers had no way to control which item stack was sent; it just picked the largest. Now each item can be dispatched manually using the buttons on the right.