[kaccess] Improve the port away from kdelibs4support

Summary:
Follow-up to 1ded47814a614debefd465cf8f962db4b8163822

• for proper D-Bus well-known name set app data before registering
• QT_QPA_PLATFORM better set before creating the QApp instance

Reviewers: Plasma, apol, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10955