Add id to search button div in menubar

[zinloosg]

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Is your feature request related to a problem? Please describe.
The introvox addon targets css selectors to explain the items on the Nextcloud dashboard to new users. All items in the menu have an ID except the search button. Adding an ID will make it easier to language independant target the search button.

Describe the solution you'd like
Add an ID to the unified search div.

Describe alternatives you've considered
Trying other selectors didn't have the effect one wants.

Additional context

[provokateurin]

Can you just open a PR to add this?