CustomWizardPage: Connect user fields to completeChanged()

Change-Id: I0000c18ffebbcb25bb203dc16fd78b804de19042
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
CustomWizard: Add a placeholder field for QLineEdit

The XML file now has a placeholdertext attribute that sets
the place holder text for the corrsponding QLineEdit.

Change-Id: I537721a5e5be796f4f88054751e66e557e718948
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>