Add option to delete all custom level save data

To match the option to nuke all main game save data, there is also now
an option to nuke all custom level save data separately (which is just
all custom level quicksaves, along with stars for level completion). It
has its own confirmation menu too. It does not delete any levels from
the levels folder.

desktop_version/src/FileSystemUtils.h

@@ -48,5 +48,6 @@ bool FILESYSTEM_openDirectoryEnabled(void);
 bool FILESYSTEM_openDirectory(const char *dname);
 
 bool FILESYSTEM_delete(const char *name);
+void FILESYSTEM_deleteLevelSaves(void);
 
 #endif /* FILESYSTEMUTILS_H */