DashboardList: Try to load dashlets when the current pane is empty

The controller only loads dashboards/dashlets either of the given name
or of the first home/pane that can be determined, so we need to load dashlets
for panes that aren't active when creating the list. Otherwise, we will end
up with empty dashboard panes.

library/Icinga/Web/Dashboard/ItemList/DashboardList.php

@@ -23,6 +23,9 @@ class DashboardList extends ItemListControl
     public function __construct(Pane $pane)
     {
         $this->pane = $pane;
+        if (! $this->pane->hasEntries()) {
+            $this->pane->loadDashboardEntries();
+        }
 
         $this->getAttributes()
             ->registerAttributeCallback('data-icinga-pane', function () {