feature for plot's build-in legend and its toggle handle

**Is your feature request related to a problem? Please describe.**
I can't get control of plot's automatically added legend, nor legends' toggle handles for image series. They are not children or descendants of the plot item.

**Describe the solution you'd like**
Treat build-in features as real items and thus we can manage their behaviors. More precisely, for auto-added legends, set them as the plot's child if there is no other legend. Same for the legends' toggle handles but it applies for every legend, no matter if it is automatically added or not.

**Describe alternatives you've considered**
No idea yet.

**Additional context**
It comes with a problem where I try to add annotations for image series and found the annotations won't hide along with the seires. I checked the image series' 'show' statements and found the toggle handles didn't change the state. I will also raise the bug issue about this.