Toggle between showing aggregate stacked plot legend or per-plot legend

[shefalijoshi]

Stacked Plot children should optionally be able to display their own legends. Rationale: lots of overlay plots in a stacked plot = lots of colors = hard to visually parse.

This should be a new toggle-able, persisted option in the stacked plot view, with the display of legends-per-child-plot enabled by default. If child legends are displayed, the aggregate legend at the top of the stack would not be displayed; if child legends are hidden, the aggregate legend would be shown.

The option should be a available in the stacked plot inspector configuration.

[shefalijoshi]

Testing Instructions:

1. Create a Stacked plot
2. Add 2 or more telemetry endpoints to the plot (NOTE: Ensure that at least one of these has staleness information)
3. Add an Overlay plot containing several telemetry endpoints as well ((NOTE: Ensure that at least one of these has staleness information))
4. Ensure that by default all the plots contained within the Stacked plot show their own legend and the aggregate legend for the stacked plot is hidden.
5. Ensure that Staleness per contained plot shows up correctly (border, staleness icon and highlight in the legend)
6. Edit the stacked plot and toggle the Show legend per plot option in the stacked plot inspector configuration.
7. Ensure that all the plots contained within the Stacked plot Do not show their own legend and the aggregate legend for the stacked plot is visible.
8. Ensure that Staleness for plots shows up correctly (border, staleness icon and highlight in the legend) in the aggregate legend.