Tarhun/airship travel

• By opening this merge request, you agree to release your code and all other changes under the GPL 3 license and to abide by the terms set by this license.
• Migrations have been added if applicable (migrations are always necessary for removing/renaming item asset files)
• Significant changes of this merge request have been added to the changelog.

Commit b61b8120:

• Fixed issues in airship_ai.rs that resulted in invalid collision avoidance.
• Removed the dead code for resuming a route after starting the server caused by the changes to migration. Airships now start out at the approach final pos each time the world is loaded so there is no need for complicated route-resume logic.
• Added some variability to the docking sequence to make it less repetitive when viewed from the airship dock. Cleaned up some comments and removed usage of more 'new' constructors in favor of in-place construction.

Latest Commit 5162f2a7:

• Now using shared Action State between pilot_airship and fly_airship functions to added tracking of holds and slowdowns
• Added variable wait times at the dock based on holds and slowdowns.