Simplify OpenComposerJob using DBus activation.

Summary:
We could use QDBusPendingCallWatcher to keep this being async,
but I'm not sure it's worth the hassle, this won't block for long.

Test Plan: Builds. No idea how to test it.

Reviewers: vkrause, dvratil

Reviewed By: vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D22663