Apply suggestions from code review

Co-authored-by: Michał Krassowski <5832902+krassowski@users.noreply.github.com>

Author: martinRenou

schema/jupyterlab-variableInspector-settings.json

@@ -1,10 +1,12 @@
 {
-  "title": "jupyterlab-variableInspector settings",
+  "jupyter.lab.setting-icon": "ui-components:list",
+  "title": "Variable Inspector",
   "description": "Settings for the jupyterlab-variableInspector extension.",
   "type": "object",
   "properties": {
     "maxItems": {
       "type": "number",
+      "minimum": 0,
       "title": "Maximum number of items",
       "description": "The maximum number of items to show in lists/dicts etc",
       "default": 10