Cargo containers should be bays, not equipment

[Saklad5]

According to page 239 of the sixth printing of TechManual, Cargo Containers are a type of transport bay. This is a significant distinction, as it means that bay doors may be allotted to containers specifically.

It also means that MegaMekLab should allow designers to adjust the quantity of cargo containers in the "Transport Bays" tab, rather than requiring them to add containers one by one. That can become quite tedious when adding a large number of containers, and having dozens makes the "Assign Criticals" tab quite a bit harder to read.