Improve the UI for the entity query (#5022)

### What

This PR implements the minimal sets of UI touch up to make the new query
editor releasable.
**Note**: both commits best reviewed separately (first one is pure
refactor)

* Fixes #4962

#### Query DSL feature flag off

<img width="322" alt="image"
src="https://github.com/rerun-io/rerun/assets/49431240/be2b6747-5254-41c6-874f-d2cd57e8fc83">

<img width="335" alt="image"
src="https://github.com/rerun-io/rerun/assets/49431240/255d7585-186b-4f9f-8b2d-4f572ac8063c">

#### Query DSL feature flag on

<img width="324" alt="image"
src="https://github.com/rerun-io/rerun/assets/49431240/35176857-0d87-46f2-9c31-ef42d5ffecc8">

<img width="334" alt="image"
src="https://github.com/rerun-io/rerun/assets/49431240/9e7c8cdc-55d9-4088-805c-c7d68d19cd9f">


### 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/5022/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/5022/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/5022/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

- [PR Build Summary](https://build.rerun.io/pr/5022)
- [Docs
preview](https://rerun.io/preview/d7ff85fab7e650f5cc2f7967e3fa497eb8fdecb0/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/d7ff85fab7e650f5cc2f7967e3fa497eb8fdecb0/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/lib.rs

@@ -1166,7 +1166,7 @@ pub fn help_hover_button(ui: &mut egui::Ui) -> egui::Response {
 }
 
 /// Show some markdown
-pub fn markdownm_ui(ui: &mut egui::Ui, id: egui::Id, markdown: &str) {
+pub fn markdown_ui(ui: &mut egui::Ui, id: egui::Id, markdown: &str) {
     use parking_lot::Mutex;
     use std::sync::Arc;