LdapUrl: fix LDAP queries broken with non-ascii search strings.

Summary:
Qt5 made query() return a PrettyDecoded query string, while the code
was calling fromPercentEncoding for the filter, i.e. assuming it to be FullyEncoded.

With unittest

Test Plan: Typing Valérie in kmail composer now finds LDAP completion correctly.

Reviewers: dvratil, mlaurent

Reviewed By: dvratil

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D12387