Even though having more than one Archive instance around is harmless, it
wastes resources with no purpose.

We thus guard the Archive pointer in Part::openFile() with a
QScopedPointer so that if one is trying to create an archive whose type
is read-only for Ark (ie. .gz files) and then chooses another mimetype,
the first created instance is deleted right away, and not when
ArchiveModel is destroyed.

ArchiveModel::m_archive is now also a QScopedPointer, so that we do not
need to manually delete the current archive when
ArchiveModel::setArchive() is called.