Wizards: add option to show an image for description

IWizard::descriptionImage() can return the path to an image.
An empty string is interpreted as no image available.

Change-Id: Ia2012eecbfdeb9bec123ed666fff2d73d79e05ce
Reviewed-by: Alessandro Portale <alessandro.portale@nokia.com>

src/plugins/vcsbase/basecheckoutwizard.cpp

@@ -119,6 +119,11 @@ QString BaseCheckoutWizard::id() const
     return d->id;
 }
 
+QString BaseCheckoutWizard::descriptionImage() const
+{
+    return QString();
+}
+
 void BaseCheckoutWizard::setId(const QString &id)
 {
     d->id = id;