Give path access and connectivity more meaning

[mlange-42]

Currently, some buildings "kind of" work, even without path access. E.g. castles and towers work as usual without paths. Warehouses provide storage, although not accessible by haulers. Mills give population support with a single road tile next to them, not taking into account any connectivity.

Ideas to make paths and connectivity more important:

• Warehouses: check connectivity -> only the largest "connected component" of warehouses counts towards the global storage capacity
• Military buildings -> must be connected to a warehouse to work?
• Mills -> must be connected to a warehouse to work?