Replace recursive isDirHidden with iterative one, allow const argument

Summary:
The non-unix implementation modifies its QDir argument to walk up the
tree. Create a copy instead.
The copy cost is amortized by no longer having to reset the dir in the
calling code, for the unix-code it is a net win.

Reviewers: Baloo, Frameworks, poboiko, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D18825