Implement Navigation using Recast/Detour

[Maxwell175]

Issue description

This PR implements Navigation starting from the work that was done on the gsoc/2020 branch.

Solution description

All commits have been rebased on master.

Checklist

I have done my best to ensure that…

• [x] …I have familiarized myself with the CONTRIBUTING.md file
• [x] …this change follows the coding style and design patterns of the codebase
• [x] …I own the intellectual property rights to this code
• [x] …the intent of this change is clearly explained
• [x] …existing uses of the Panda3D API are not broken
• [ ] …the changed code is adequately covered by the test suite, where possible.