Conditional imports

In order to restrict applications or components in general, a security mechanism is needed in Qt. The basic design should be the following:

In the import code path, the import is either done right away (current case), or it uses an optional import policy agent, which checks if the application or component has sufficient privileges, and either allows or denies the import
The policy agent is replacable, much like QNetworkAccessManager
import performance in current case should not suffer