fix: memory leak in menubar

[SimonSiefke]

Fixes #198051

Output with --measure instance-counts

{
  "instanceCounts": {
    "before": [
      {
        "name": "Menu",
        "count": 1
      }
    ],
    "after": [
      {
        "name": "Menu",
        "count": 1
      }
    ]
  },
  "isLeak": false
}

[bpasero]

I think from here on I have to defer to Steven to review because I am not the original author of the code. Its not clear to me all the paths that lead to re-create or dispose of the custom menu...

Specifically, there seems to be calls to showCustomMenu without cleanupCustomMenu, so not clear to me if we still leak.

resolved