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/plugin.json

@@ -2,5 +2,6 @@
     "name": "Navigation",
     "version": "1.0.0",
     "author": "CodePress",
-    "description": "Essentiële navigatie plugin voor handleidingen en content"
+    "description": "Essentiële navigatie plugin voor handleidingen en content",
+    "type": "content"
 }