[calamares] Once the steps are loaded, activate the first view step

- Using next and back buttons calls onActivate() on the view step
   that you end up on.
 - The first view step to be shown, though, doesn't get an onActivate()
   (unless you go, say, next and then back).
 - Explicitly call onActivate() on the first view step once they're
   all loaded.

FIXES #1156

src/calamares/CalamaresApplication.cpp

@@ -363,6 +363,12 @@ CalamaresApplication::initViewSteps()
     ProgressTreeModel* m = new ProgressTreeModel( nullptr );
     ProgressTreeView::instance()->setModel( m );
     cDebug() << "STARTUP: Window now visible and ProgressTreeView populated";
+
+    const auto steps = Calamares::ViewManager::instance()->viewSteps();
+    cDebug() << Logger::SubEntry << steps.count() << "view steps loaded.";
+    // Tell the first view that it's been shown.
+    if ( steps.count() > 0 )
+        steps[0]->onActivate();
 }
 
 void