Utils: Introduce a Layouting::empty item

A bit more explicit than the current mix of Space() and asserting {}.

Change-Id: Ia296b1d23294dfccd1e33827e136b821a4d2c6fe
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/libs/utils/filewizardpage.cpp

@@ -75,7 +75,7 @@ FileWizardPage::FileWizardPage(QWidget *parent) :
     using namespace Layouting;
 
     Form {
-        Space(), d->m_defaultSuffixLabel, br,
+        empty, d->m_defaultSuffixLabel, br,
         tr("File name:"), d->m_nameLineEdit, br,
         tr("Path:"), d->m_pathChooser
     }.attachTo(this);