IWizard and derived classes: Code cleanups

There should be no functional changes, just removal of unnecessary
code. This includes:

* Removal of unused QObject *parent = 0 parameters to constructors
* Removal of unnecessary explicits
* Removal of unnecessary virtuals
* Removal of unnecessary constructors/destructors

Some explicits were added though where those were missing.

Change-Id: Iab570349ea950dad0a2d01af17bc6175f70832f1
Reviewed-by: Eike Ziller <eike.ziller@digia.com>