Make Dolphin's SortFilterProxyModel for KDirModel public

[Nick Shaforostoff]

>I'm not sure if KIO may have a NEPOMUK dependency
It doesnt have to. You (and I) can remove all #if 0 and #if HAVE_NEPOMUK_NONO blocks.
I merely suggest that nepomuk part of the filter be moved to kdelibs/nepomuk as a separate class (from dolphin)
in kde 4.1 (because we wanna release usable KDE as early as possible, and such nepomuk code can wait).