RecastDemo: add Load & Save buttons for SoloMesh Sample, and fix warnings (#258)

* 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.

RecastDemo/Source/InputGeom.cpp

@@ -92,6 +92,7 @@ static char* parseRow(char* buf, char* bufEnd, char* row, int len)
 			case '\t':
 			case ' ':
 				if (start) break;
+				// else falls through
 			default:
 				start = false;
 				row[n++] = c;