Show visualization usage

[ChrisJNL]

It would be great to have an option in settings page to show if a visualization is curently used in a dashboard.

Indeed, i create a lot of visualization, but the only useful page (for me) is the dashboard one.
However, i dunno how to easily delete old visualization.

An indicator ( ie (1) after the name) to show how many dashboards use this visualization could be helpful :)

[timroes]

I will close this in favor of #16912 and #5947 since they touch pretty much the same topic.