Make Dolphin's SortFilterProxyModel for KDirModel public

[Nick Shaforostoff]

>Where you guys thinking of libkio or libkfile? I guess it belongs to libkfile if most
>non-filemanagement-related applications are going to use that class only through the file dialog.
I've put it in kio because that's where kdirmodel.cpp reside (and the proxymodel can be used only with it)

there [essentially] only 1 filedialog and >2 apps that use kdirmodel...

I will also update API docs for KDirModel.