Add KFileCustomDialog constructor with a startDir parameter.

Summary:
This is needed because KFileWidget does special handling of the startDir
parameter (to detect if it's directory+filename), while setUrl() assumes
it's a directory.

Test Plan: tested with D26645 in kolourpaint

Reviewers: broulik, pino, mlaurent

Reviewed By: mlaurent

Subscribers: alexeymin, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26701