Fix the total mess with single/double click in KDirOperator:
- make KDirOperator honor the system single/double click setting by default
- make the single/double click behavior logical:
single click selects files and directories
double click higlights files and directories
this 1:1 follows KDE3 behavior

- unbreak file/directory activation from keyboard:
return press on file selects file
return press on directory sets the URL

this finally fixes the bug in KFileWidget when you could not navigate into directory with keyboard - the widget used to select directory instead of going into it. this also fixes similar keyboard navigation bug in Kate file selector again, this closely follows KDE3 behavior

- change setOnlyDoubleClickSelectsFiles() method API doc to say what it really does - changes the signle/double click mode despite the system setting remove the word "hack" from API doc - it's not really the word we need there ;)