Ability to "pin" a selection to make it persistent

[cdmihai]

Sometimes I want to track multiple selections, each selection with a different color. It would be awesome if:

• Like today, I do a selection which colors all the instances.
• I right click on a selection and choose "Pin selection". This has the effect that if I cancel the current selection (click somewhere else, or select make a new selection), the pinned instances remain colored.
• Given multiple existing pinned selections, when I pin a new selection it chooses a background color different from the existing pinned colors. This ensures each new pinned selection has a different color. I don't think people would pin more than 2-4 selections, so the space of easily distinguishable colors won't get exhausted.
• Removing a pinned selection: right click and select "Unpin".
• Removing all selections: either refresh the page or right click any pinned selection and select "Unpin all"

Detailed use case: I often feel the need to do this when reading code. I have a symbol of interest (variable, method call, etc) and I either want to see how it correlates with another entity (when does this variable get passed to this method?) or I want to quickly understand something else, like a nested understanding session where I pause my current search and do another one: I want to keep my initial selection to return back to it. Both of these tasks can be made way easier if I could pin two or more selections.