QtIFW

Revert parts of the former changes.

Now reset the core checked list in show event, as this is
not called that frequently as nextId(). Solves the problem
of always recalculating the components to install.

Change-Id: I282d25236441c6a91c833867ce65aa351dcfcc67
Reviewed-on: http://codereview.qt.nokia.com/3503
Reviewed-by: Tim Jenssen <tim.jenssen@nokia.com>