[PATCH] shell: Rearrange the teardown order

[PATCH] shell: Rearrange the teardown order

At the moment, the ShellCorona is destroyed after the QApplication object.
Destroying something after the application object can lead to
unexpected results because most of the code is written with an assumption
that the app object and the associated objects, e.g. the qpa, are still
valid when the cleanup code runs.

This change puts the ShellCorona on the stack so the destruction order
looks as follows:

- destroy ShellCorona
- destroy QApplication

CCBUG: 487660

(cherry picked from commit e2326d7f9e752eb18411ef4c0bcd53b8f34e02c6)

Co-authored-by: Vlad Zahorodnii <vlad.zahorodnii@kde.org>
Gbp-Pq: Name upstream_88911e82_shell-Rearrange-the-teardown-order.patch