qml: Fix potential memory leak in the file selector

The myInstance doesn't seem to be part of a QObject parent/child
relationship and Coverity complained about it. Use a QScopedPointer
to make sure the data will be deleted.

Fixes: CID 10606

Change-Id: I30848ab4d1e621d82281aaa7d6f84e0c2cce4084
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>