Details view: Allow to turn off expandable folders like in Dolphin 1.7

The option for turning off expandable folders has been removed with
the new view-engine. Due to several requests this option has been
readded again.

As for 4.8.x no new user-interface strings may be introduced, the
line
  ExpandableFolders=false
must be manually added below the section "[DetailsMode]" in the file
~/.kde/share/config/dolphinrc if the expandable-folders feature
should be disabled.

Thanks to H.H. "cyberbeat" for the initial patch!

BUG: 289090
FIXED-IN: 4.8.2