Support for a Montage dashboard (custom collections of graphs)

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: For big organization with numerous dashboards, we sometimes need to laterally compare graphs side by side but those might be located in different dashboards. Even if they're on the same board, it's a bit cumbersome to scroll the page up and down multiple times to do comparison. **What would you like to be added**: Would be ideal if we can dynamically / temporarily pull graphs we're interested into a common place, so they're easy to do collateral comparing, especially for incident troubleshooting and report generating. Chose the word `Montage` because visually they resemble a lot. **Who is this feature for?** Any grafana user could benefit from this feature.