Optimize Baloo::File copy assign operator, fix Baloo::File::load(url)

Summary:
There is no need to delete and create a new Private on assignment, it is
sufficient to reassign its members.

File::load(url) resets the url, afterwards he old corresponding property
map is no longer valid, clear it.

Test Plan: ctest

Reviewers: Baloo, Frameworks, ngraham, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D18877