Plugin system: content vs system types, sidebar-aware UI, arrow visibility

- plugin.json type field: 'content' (sidebar) or 'system' (API only)
- Content-edit: label 'Plugins', hide section if layout has no sidebar
- Content-edit: disable checkboxes when no sidebar layout selected
- Content-edit: hide up arrow on first item, down arrow on last item
- PluginManager: isPluginViewable() checks type=system -> not viewable
- Admin plugins page: show Content/Systeem type badge
- HTMLBlock: add plugin.json with type=content
- Navigation: add type=content to config

plugins/Navigation/Navigation.php

@@ -10,6 +10,7 @@ class Navigation
         $this->config = [
             'title' => 'Navigatie',
             'viewable' => true,
+            'type' => 'content',
         ];
     }