Prefer SecretService if avialable for KDE

[Thaodan]

The user might have installed another service or use Kwallet with SecretService support enabled.

References:

• https://bugs.kde.org/show_bug.cgi?id=313216#c36

[michaelk83]

PR is at #221 (to link it properly here), but there are some blockers. See the blockers list in KDE bug 458318.