* Implement saveAll loadAll for Sample_SoloMesh
- Solo mesh sample is loaded and saved as a tiled mesh containing
an unique tile, so as to remain compatible with tiled mesh
- navmesh is saved to/loaded from `solo_navmesh.bin` binary file
- add and connect the corresponding 'Save/Load' GUI elements
* Fix `implit-fallthrough` gcc warnings
On linux with gcc 7.2.0+ and all warnings treated as errors,
implicit fallthroughs in case statements are considered as errors.
If the comment just below the next `case` or `default` matches
various strings, one of which is `falls through` the fallthrough
is then considered explicit.
* Added virtual qualifier to a few destructors in subclasses.
* Removed a few empty destructors that didn't need to be explicitly defined.
* Fixed a few typos
* Removed unnecessary empty virtual destructors in some classes with parent's who derrived from a class with a virtual destructor
