Respect last-show-hidden during search (Issue #785)

If “show hidden” is switched off, do not visit hidden subfolders and do not
return hidden files. You can still start the search from within a hidden
folder.

While viewing search results, restart the search if show_hidden changes via
the window action. This will happen much more rarely than changing the
search string, so it’s OK to reset the search here too.

MR !246