fix: memory leak in extension icon widget

[SimonSiefke]

Fixes a memory leak in extension icon widget by using addDisposableListener(this.iconElement, 'load' instead of this.iconElement.onload.

Before

When toggling the extension search view 37 times, the number of load event listeners grows to 185:

{
  "eventListenersWithStackTrace": [
    {
      "type": "load",
      "description": "() => this.iconElement.style.visibility = \"inherit\"",
      "objectId": "4623829927015327231.1.34282",
      "stack": [
        "listener (vscode/out/vs/workbench/contrib/extensions/browser/extensionsWidgets.js:125:36)",
        "ExtensionIconWidget.render (vscode/out/vs/workbench/contrib/extensions/browser/extensionsWidgets.js:126:35)",
        "ExtensionIconWidget.update (vscode/out/vs/workbench/contrib/extensions/browser/extensionsWidgets.js:70:10)",
        "set extension (vscode/out/vs/workbench/contrib/extensions/browser/extensionsWidgets.js:67:10)",
        "vscode/out/vs/workbench/contrib/extensions/common/extensions.js:57:48",
        "Array.forEach (<anonymous>)",
        "set extension (vscode/out/vs/workbench/contrib/extensions/common/extensions.js:57:21)",
        "set extension (vscode/out/vs/workbench/contrib/extensions/browser/extensionsList.js:141:39)",
        "Renderer.renderElement (vscode/out/vs/workbench/contrib/extensions/browser/extensionsList.js:175:20)"
      ],
      "count": 185,
      "originalStack": ["src/vs/workbench/contrib/extensions/browser/extensionsWidgets.ts:131:31"],
      "originalName": null
    }
]

After

No more load event listener leak detected