Fix external application menu occasionally slowing down startup

Summary:
If moving the menubar entries to a button in the window decoration is
enabled, the bug reporter experiences delays of up to 25 seconds in the
startup of Gwenview.

Bisecting points to 9631043c, which added MPRIS support through
providing a D-Bus interface.

While the exact nature of the problem remains unclear due to not being
reproducible outside of the bug reporter's environment, moving the
initialization of the MPRIS support after `createGUI` is reported to
correct the issue. This may be due to avoiding conflicts or races with
the D-Bus connection, which is used for the MPRIS service, the
ScreenSaver service and the application menu integration.

Thanks to Duncan for reporting the bug and coming up with a fix.

BUG: 395925
FIXED-IN: 18.04.3

Test Plan:
`dbus-send --session --dest=org.mpris.MediaPlayer2.Gwenview --type=method_call /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player.Next` still moves to the next image in Gwenview.
The external application menu is still working fine.

Reviewers: #gwenview, kossebau

Subscribers: broulik

Differential Revision: https://phabricator.kde.org/D13995