Clean up the textures when deleting the content

Whenever the content is deleted then the textures will be recreated so the
existing list of textures should be deleted and cleared to reclaim the
memory.

Change-Id: I4fdf77817a5f4b2330414e9d113c669d18de9af8
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
QQmlComponent::create(): visual items must have a visual parent.

There seems to be a bit of confusion about this on public forums.

Change-Id: Id193de541e7d7e353dc5d75b64a15f481e2cf8b6
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>