Add navmesh editor based on Walkabout

[crabmusket]

For 3.7 #1092

• [x] Initial merge
• [x] Remove silly GUI container class
• [x] Redo editor icon and cover mode icon
• [x] Fix issue with scaled ConvexShapes objects
• [x] #956
• [x] #784
• [x] Rename all instances of Walkabout

  For 3.x

• [ ] Investigate console errors about namespace linking
• [ ] #695
• [ ] Investigate memory leak when rebuilding tiles (reproduce with this)
• [ ] Render using vertex buffers like a normal person
• [ ] Port docs to wiki

[Azaezel]

https://gist.github.com/Azaezel/11591952400696eb7d79 is a minimum reproduction case for the memleak. Again, I suspect it's simply a case of triggering rebuilds so quickly it's stepping on itsself.

[Azaezel]

resolved