Pathfinding

[subchannel13]

Add pathfinding service that suggests shortest path when manually sending fleets and is used by transport automation.

• [x] Pathfinding interface
• [ ] Cache previous shortest path calculation
• [x] Pathfind used by transport automation
• [x] Pathfinding used when manually sending a fleet
• [x] Way to force direct flight

[subchannel13]

Caching is postponed if/when it becomes necessary.