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