Abstract assets mounting to FileSystemUtils.cpp

The assets mounting code was put directly in editorclass::load(), but
now it's in a neat little function so it can be called from multiple
places without having to call editorclass::load().

desktop_version/src/FileSystemUtils.h

@@ -15,6 +15,7 @@ char *FILESYSTEM_getUserLevelDirectory();
 
 bool FILESYSTEM_directoryExists(const char *fname);
 void FILESYSTEM_mount(const char *fname);
+void FILESYSTEM_mountassets(const char *path);
 void FILESYSTEM_unmountassets();
 
 void FILESYSTEM_loadFileToMemory(const char *name, unsigned char **mem,