Introduce KFileItemListProperties::isFile()

Summary:
This method can be used to check whether a list of selected
URLs contains only files (and zero folders).

The implementation matches the isDirectory() method and relies
on the values returned by KFileItem::isFile().

Unit test is updated accordingly.

Reviewers: Frameworks, dfaure

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12727