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

cms/core/plugin/PluginManager.php

@@ -132,7 +132,13 @@ class PluginManager
     {
         if (method_exists($plugin, 'getConfig')) {
             $config = $plugin->getConfig();
-            return !isset($config['viewable']) || $config['viewable'] !== false;
+            if (isset($config['viewable']) && $config['viewable'] === false) {
+                return false;
+            }
+            // System plugins don't show in sidebar
+            if (isset($config['type']) && $config['type'] === 'system') {
+                return false;
+            }
         }
         return true;
     }