Improve single-instance application behavior

Summary:

1. Avoid showing the window of the second instance by creating KDBusService object before creating the window object.
2. Activate the window on the second start.

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: mart, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12760