Introduce structs for dynamically resolved functions per DLL.

Preparing the adding of the iconic preview functions; the current
approach of having one qt_ .. function does not scale well.

Task-number: QTBUG-36730
Change-Id: Id70b9b13b8cd09d4d9c2830e9d6d0a413fd812a3
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>