First stage in converting the Irrlicht interface to a library:

- move all files to src/unit_IRRLICHT
- add `ChApiIrr.h` for definition of export/import symbols
- add stub source file `ChIrrApp.cpp`
- modify/add `CMakeLists.txt` files
- remove obsolete `ChPovTools.h`
Change path to unit_IRRLICHT headers in demos.
Add custom command to copy the Irrlicht DLL as a post-build step.
Link ChronoEngine_IRRLICHT library to demos.
Split ChIrrAppInterface into header and implementation files.
Replace `ChDisplayTools.h` with `ChIrrTools.{h,cpp}` header and implementation files.
Split `ChIrrNodeProxyToAsset` into header and implementation files.
Split `ChIrrCamera` into header and implementation files.
Split `ChIrrAssetConverter` into header and implementation files.
Update FEM demos to use the ChronoEngine_IRRLICHT library.
Split `ChIrrMeshTools` into header and implementation files.

Convert to free functions.
Split `ChBodySceneNode` into header and implementation files.
Split `ChBodySceneNodeTools` into header and implementation files.
Include required header in demo_tracks.
Split `ChIrrNode` into header and implementation files.
Rename `ChParticlesSceneNode` and split into header and implementation files.
Split `ChIrrWizard` into header and implementation files.
Move implementation of ChIrrApp methods to cpp file.
Merge branch 'develop' into feature/irrlich_lib
Update the FindChronoEngine CMake script:

- rename to `FindChronoEngine.cmake`
- if unit_IRRLICHT component requested, find the ChronoEngine_IRRLICHT interface library
- proceed without an error if we cannot find both Release and Debug libraries, provided we find at least one set.