Context Menu 1: Basic scaffolding and simple actions (#5163)

### What

* Part of #4823

This PR is the first of a series to implement context menus in the
Blueprint Tree. Here we setup the basic scaffolding and implement some
basic actions:
- Add SV
- Add container
- Show/hide
- Remove

<img width="444" alt="image"
src="https://github.com/rerun-io/rerun/assets/49431240/58ad8d62-ea69-47c6-8e88-844c611c4832">

<br/>
<br/>

The selection is currently ignore and the actions only apply to the
clicked item. TODO in future PRs:
- improved (multi-)selection handling
- additional actions
- entity level manipulations
- use ListItem instead of label/WidgetText
- better folder structure
- release checklist update

### 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/5163/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/5163/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/5163/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/5163)
- [Docs
preview](https://rerun.io/preview/4b5e47083d3aa129352f0fbec9a74db500e20f16/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/4b5e47083d3aa129352f0fbec9a74db500e20f16/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

Cargo.toml

@@ -74,27 +74,27 @@ re_ws_comms = { path = "crates/re_ws_comms", version = "=0.14.0-alpha.1", defaul
 rerun = { path = "crates/rerun", version = "=0.14.0-alpha.1", default-features = false }
 
 # egui-crates:
-ecolor = "0.26.1"
-eframe = { version = "0.26.1", default-features = false, features = [
+ecolor = "0.26.2"
+eframe = { version = "0.26.2", default-features = false, features = [
   "accesskit",
   "default_fonts",
   "puffin",
   "wayland",
   "x11",
 ] }
-egui = { version = "0.26.1", features = [
+egui = { version = "0.26.2", features = [
   "callstack",
   "extra_debug_asserts",
   "log",
   "puffin",
   "rayon",
 ] }
 egui_commonmark = { version = "0.12", default-features = false }
-egui_extras = { version = "0.26.1", features = ["http", "image", "puffin"] }
-egui_plot = "0.26.1"
+egui_extras = { version = "0.26.2", features = ["http", "image", "puffin"] }
+egui_plot = "0.26.2"
 egui_tiles = "0.7.2"
-egui-wgpu = "0.26.1"
-emath = "0.26.1"
+egui-wgpu = "0.26.2"
+emath = "0.26.2"
 
 # All of our direct external dependencies should be found here:
 ahash = "0.8"