[libcalamaresui] Fix disable-cancel behavior

- d78bc0c5 added an early `return false` when cancel is disabled,
   before checking if we were at the last step; so last-step
   didn't get any special handling.
 - refactor so that last-step now gets special handling first,
   **then** disable-cancel handling, and then the usual case.

src/libcalamaresui/ViewManager.cpp

@@ -367,15 +367,17 @@ bool ViewManager::confirmCancelInstallation()
 {
     const auto* const settings = Calamares::Settings::instance();
 
+    // When we're at the very end, then it's always OK to exit.
+    if ( m_currentStep == m_steps.count() -1 && m_steps.last()->isAtEnd() )
+        return true;
+
+    // Not at the very end, cancel/quit might be disabled
     if ( settings->disableCancel() )
         return false;
     if ( settings->disableCancelDuringExec() && stepIsExecute( m_steps, m_currentStep ) )
         return false;
 
-    // If it's NOT the last page of the last step, we ask for confirmation
+    // Otherwise, confirm cancel/quit.
-    if ( !( m_currentStep == m_steps.count() -1 &&
-            m_steps.last()->isAtEnd() ) )
-    {
     QString title = settings->isSetupMode()
         ? tr( "Cancel setup?" )
         : tr( "Cancel installation?" );
@@ -395,9 +397,6 @@ bool ViewManager::confirmCancelInstallation()
     int response = mb.exec();
     return response == QMessageBox::Yes;
 }
-    else // Means we're at the end, no need to confirm.
-        return true;
-}
 
 void
 ViewManager::updateCancelEnabled( bool enabled )