Add an explicit DataResult variant to re_viewport_context::Item

[abey79]

What

Basically this:

   pub enum Item {
       /* ... */
-      InstancePath(Option<SpaceViewId>, InstancePath),
+      InstancePath(InstancePath),
+      DataResult(SpaceViewId, InstancePath),
   }

The motivation is to make the distinction between an entity/instance from the data store distinct from an entity/instance in the context of a space view query's data result.

Note: I initially considered having an InstancePath instead of an EntityPath for Item::DataResult. However, after discussing with @Wumpf, we deemed that undesirable as we want to be able to select (or otherwise consider) single instances from data results.

This PR also fixes a very subtle usability fix whereby clicking on the shown button in the selection panel for a data store instance would lead to the corresponding space view data result entity (not instance!). This is fixed now.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
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  • Using newly built examples: app.rerun.io
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[Wumpf]

neat