Help browser: add an index cache to speed up launch times.

Features:

- Rebuilds and caches the search index if there is no cache yet, or if
  the cache is outdated (i.e., any cached directory or its parent was
  removed or modified since the cache was last created).

- Keeps track of the modification times of all cached directories of
  help files, as well as their parent directories, in order to determine
  when the cache is outdated. The parents are tracked to catch changes
  where new sibling directories are added. This may produce some false
  positives and still isn't 100% foolproof, but it is as close as we can
  get if we still want to achieve substantial speedups.

- Automatically rebuilds the index (and cache) from scratch, as soon as
  the help browser is relaunched after changes to the browser
  configuration in the gui prefs. So there's no need any more to
  relaunch Purr Data to have these changes take effect.

Both the index cache and the directory timestamps are maintained as
ordinary text files with a straightforward syntax (basically
colon-delimited csv) in the user's home directory. The files are located
in configuration directories (.purr-data on Linux and Mac,
AppData/Roaming/Purr-Data on Windows). Thus it's easily possible to
modify these files with external tools (e.g., if we want to upgrade the
file format in the future.)