Dashboard variable: focus on selected value when reopening dropdown

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: When selecting a value in the variable dropdown, instead of focusing on the previously selected value, the dropdown stays at the top. When there are many values in the variable, this makes it difficult to select the "next" value (the one next to the previously selected one). **What would you like to be added**: An option in the settings to "focus" on the currently selected value when opening the dropdown. **Who is this feature for?** Dashboard administrators and dashboard users.