[libcalamaresui] Disallow closing the window during execution

- If the disable-cancel-during-exec setting is on, and the user clicks
   the window-close button, then disregard the close message.

src/libcalamaresui/ViewManager.cpp

@@ -373,6 +373,8 @@ bool ViewManager::confirmCancelInstallation()
     
     if ( settings->disableCancel() )
         return false;
+    if ( settings->dontCancel() && stepIsExecute( m_steps, m_currentStep ) )
+        return false;
     
     // If it's NOT the last page of the last step, we ask for confirmation
     if ( !( m_currentStep == m_steps.count() -1 &&