Update Navigation

[berndrakell]

Closes #74

• Buildings and resources are now included in the navigation mesh. This improves unit pathfinding.
• Each fixed structure/resource now has an invisible mesh that belongs to the node group which is taken into account for navigation
• Also the navigation radius of the tank was lowered to make it more manoeuvrable and close the gap between big and small units since the NavMesh is baked for both of them but orients itself on the bigger units
• Because of how the map generation is implemented in the game there occurred some weird situations where the initial buildings floated and got relocated. To fix this I made the initial NavMesh baking ignore the buildings, and then after a wait frame the mesh is updated. I honestly don't completely understand what happens in the test games setups so my solution is a bit hacky
• Although I'm not completely happy with the implementation of the solution, I think pathfinding is now much better

[Scony]

I haven't checked the details, but the description looks interesting. Thanks for opening this one! And thank you for keeping the PR <200LOC - it will make the review doable, although it will take some time.

[Scony]

I've checked the changes and made some notes - in principle the PR looks well. I see some room for improvement and experiments, therefore I'll merge it locally, and make some changes on top of that.

[Scony]

Merged and reworked, thanks! Please note that I've hidden that feature behind feature flag as I need to fix few more things that popped up.