applicationicons.svg: Use one shared icon background

...instead of duplicating it. This way we can apply a unified outline
to the Qt logo to work around scaling aritefacts on OSX.

Task-number: QTCREATORBUG-13359
Change-Id: I23ababf0a6598d92682e65856676b113937c86af
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>