[PathFinding] Refactor

[D8H]

(this PR is based on the PR for isometry, so it can't be merged, it's just to open the discussion)

This is a custom build of GDevelop 5 beta 110 with this change:
https://www.dropbox.com/s/mp5sry99aflw9yv/gdevelop-iso-path-beta110-2021-05-16.7z?dl=1

I did some refactoring on the path finding behavior.
I split a code that was in a mono-block into this:

• AStarPathFinder is the path finding algo alone
• NodeEvaluator says which node is passable and it's cost
• GridGraph is the graph data structure
• Grid hold the grid model:
  how nodes must be linked to one another
  coordinates <-> node index mapping

The whole thing is independent from the PathFindingRuntimeBehavior so it will allow to write new extensions based on the path finding.

• the grid interface allows to implements other grids like hexagonal grids for instance.
• the NodeEvaluator allows to define what make a good path. So other strategy can be implemented. For instance, let say you want to implement an extension for enemies that avoid light you would only have to implement a new evaluator that tweak the cost and rely on the existing one for collisions.