Use QPointer member variables in Gwenview StartHelper

Summary:
cppcheck complains about the never deleted local pointer variable
that creates the Gwenview::MainWindow.
This patch uses QPointer and QScopedPointer for MainWindow
and the optional QTemporaryDir variables to clean this.

Test Plan:
Check Gwenview's normal behaviour starting with different arguments
(none, single/multiple image/s, single/multiple directory/ies).

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D10964