GUI: The "Overwrite" button when a directory already exists has always confused people, name it "Write Into" instead, with tooltips that explain what the 4 buttons do in both cases (files or dirs); separate the effect of those buttons so that "write into all dirs" doesn't imply "overwrite all files" and vice-versa; same with skipping files vs skipping dirs.

As discussed with JohnFlux and rabauke on irc; fixes #84752.