testbed for UI (#18091)

# Objective

- have a testbed for UI

## Solution

- move previous `ui` example to `full_ui`
- add a testbed ui with several scenes
- `ui_layout_rounding` is one of those scenes, so remove it as a
standalone example

the previous `ui` / new `full_ui` is I think still useful as it has some
things like scroll, debug ui that are not shown anywhere else

.github/example-run/testbed_ui.ron

@@ -1,6 +1,4 @@
 (
     events: [
-        (100, Screenshot),
-        (200, AppExit),
     ]
 )

Cargo.toml

@@ -4168,11 +4168,11 @@ doc-scrape-examples = true
 hidden = true
 
 [[example]]
-name = "testbed_ui_layout_rounding"
-path = "examples/testbed/ui_layout_rounding.rs"
+name = "testbed_full_ui"
+path = "examples/testbed/full_ui.rs"
 doc-scrape-examples = true
 
-[package.metadata.example.testbed_ui_layout_rounding]
+[package.metadata.example.testbed_full_ui]
 hidden = true
 
 [[example]]