Refactor Selection using IndexMap and make it more encapsulated

[abey79]

What

This PR change Selection to use an IndexMap<Item, Option<SelectedSpaceContext>> instead of a Vec<(Item, Option<SelectedSpaceContext>). This is a much better model for the selection that we currently have (Item may not be duplicated, even with different space context) and paves the way for a clean "remove" API (needed by #5465).

The choice of IndexMap is motivated by keeping the selection in the click-order, as was previously the case with Vec. This was a pre-existing transitive dependency.

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[Wumpf]

nice. IndexMap seems to work just as well and is not a new dependency (in the grand picture of things), so why not :)