Do not use the object pool to hold potential wizards. Register
FactoryCreator functions with IWizardFactory instead and use
those to create the wizards when necessary.

This saves us a couple of cycles during startup since we can now
delay construction of all wizards and it makes us more flexible
wrt. managing the lifecycle of the wizard factories.

Change-Id: I95d6a6dfcdf0fd995e1934a9fefcd96c6a676753
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>