TargetSetupPage: Collapse widgets which the user unchecks.

Feels better that way.

Change-Id: I69f9140037a725a5e937e5d34c2f5c6eed1737dd
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
TargetSetupPage: Fix layout code for m_importWidget and m_spacer

Those should be added once at initialization time, not for each
setupWidget call.

Change-Id: Id65b919f911e47d2f687bcb3c40e7120d63c5468
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>