Allow webview panels to provide a custom icon

Allow webview panels to set an icon. This should probably be done declaratively in the `package.json`, something like:

```json
"contributes": {
    "webviews": [
      {
        "viewType": "markdown.preview",
        "icon": {
          "light": "./media/Preview.svg",
          "dark": "./media/Preview_inverse.svg"
        }
      }
    ]
  
}
```

/cc @jrieken 