Adding new slightly different icon for Struct to make it
different from Class.

Task-number: QTCREATORBUG-8574
Change-Id: I3da655adeaa05d7a590d4ec00f9749b5d86e0340
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>