Do not duplicate the space origin subtree in the blueprint tree UI (#…

…5371)

### What

Follow-up to:
- #5342

After #5342, it was possible for a data result subtree to be displayed
twice in the blueprint tree UI, for example:

<img width="778" alt="image"
src="https://github.com/rerun-io/rerun/assets/49431240/13da8014-5abf-4206-bba8-d9d5a33d3919">

<br/>

This is a problem for storing collapsed state (#5362) and is generally a
weird UX.

This PR replaces the origin subtree by a link which, when clicked,
selects the actual origin subtree (which is always displayed above the
projections).


https://github.com/rerun-io/rerun/assets/49431240/6655a187-c786-4d30-8451-e473267e4526


### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[app.rerun.io](https://app.rerun.io/pr/5371/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/5371/index.html?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[app.rerun.io](https://app.rerun.io/pr/5371/index.html?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5371)
- [Docs
preview](https://rerun.io/preview/8f4012dfde1a2d023ed2493bb7009fd342989b53/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/8f4012dfde1a2d023ed2493bb7009fd342989b53/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

crates/re_ui/src/icons.rs

@@ -140,6 +140,7 @@ pub const ENTITY_EMPTY: Icon = Icon::new(
     "entity_empty",
     include_bytes!("../data/icons/entity_empty.png"),
 );
+pub const LINK: Icon = Icon::new("link", include_bytes!("../data/icons/link.png"));
 pub const COMPONENT: Icon = Icon::new("component", include_bytes!("../data/icons/component.png"));
 
 pub const STORE: Icon = Icon::new("store", include_bytes!("../data/icons/store.png"));

crates/re_viewport/src/viewport_blueprint_ui.rs

@@ -288,6 +288,7 @@ impl Viewport<'_, '_> {
                     &DataResultNodeOrPath::from_path_lookup(result_tree, &space_view.space_origin),
                     space_view,
                     space_view_visible,
+                    false,
                 );
 
                 // Show 'projections' if there's any items that weren't part of the tree under origin but are directly included.
@@ -317,6 +318,7 @@ impl Viewport<'_, '_> {
                             &DataResultNodeOrPath::DataResultNode(projection),
                             space_view,
                             space_view_visible,
+                            true,
                         );
                     }
                 }
@@ -357,10 +359,32 @@ impl Viewport<'_, '_> {
         node_or_path: &DataResultNodeOrPath<'_>,
         space_view: &SpaceViewBlueprint,
         space_view_visible: bool,
+        projection_mode: bool,
     ) {
         let store = ctx.entity_db.store();
 
         let entity_path = node_or_path.path();
+
+        if projection_mode && entity_path == &space_view.space_origin {
+            if ListItem::new(ctx.re_ui, "$origin")
+                .subdued(true)
+                .italics(true)
+                .with_icon(&re_ui::icons::LINK)
+                .show(ui)
+                .on_hover_text(
+                    "This subtree corresponds to the Space View's origin, and is displayed above \
+                    the 'Projections' section. Click to select it.",
+                )
+                .clicked()
+            {
+                ctx.selection_state().set_selection(Item::InstancePath(
+                    Some(space_view.id),
+                    InstancePath::entity_splat(entity_path.clone()),
+                ));
+            }
+            return;
+        }
+
         let data_result_node = node_or_path.data_result_node();
 
         let item = Item::InstancePath(
@@ -460,6 +484,7 @@ impl Viewport<'_, '_> {
                                 &DataResultNodeOrPath::DataResultNode(child_node),
                                 space_view,
                                 space_view_visible,
+                                projection_mode,
                             );
                         }
                     },