Make Dolphin's SortFilterProxyModel for KDirModel public

[Nick Shaforostoff]

we can provide sorting model for kdirmodel that:

-reimplements hasChildren and canFetchMore
-sorts dirs first
-does natural sorting

so the dirtree sorting will be consistent over all kde

it could be reused at least in konqueror and gwenview (thats what lxr says), and of course, i could use it in my KAider.

i can prepare it in a few days and commit it on friday night :)
any objections?