Include panel title after dashboard title in HTML title tag

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: It would make it easier to find a certain panel in the browser's history. **What would you like to be added**: At the moment, when viewing a dashboard, the HTML title tag includes the dashboard title, e.g. _Icinga2 - Grafana_. However, when viewing a specific panel, the title does not change accordingly. Would it please be possible to append the panel title to the `<title>` tag? This would result in something like _Icinga2 - **CPU usage** - Grafana_ which could be easily found in the browser histor, making it possible to quickly access this specific panel. This would also make it easier to identify multiple browser tabs open on different panels of the same dashboard. Initially requested in #39926. **Who is this feature for?** This would mostly benefit users who often view a single panel instead of a whole dashboard.