UI Display and Visibility Example (bevyengine#7629)

# Objective

An example demonstrating how Display and Visibility work in Bevy UI.

fixes bevyengine#5380
related bevyengine#5368

![Bevy App 15_02_2023
20_40_46](https://user-images.githubusercontent.com/27962798/219150865-419ade53-250b-4030-8197-907cac7aa5da.png)

## Changelog

* Added the example `flex_display.rs`.

Cargo.toml

@@ -1792,6 +1792,16 @@ description = "Illustrates creating and updating a button"
 category = "UI (User Interface)"
 wasm = true
 
+[[example]]
+name = "display_and_visibility"
+path = "examples/ui/display_and_visibility.rs"
+
+[package.metadata.example.display_and_visibility]
+name = "Display and Visibility"
+description = "Demonstrates how Display and Visibility work in the UI."
+category = "UI (User Interface)"
+wasm = true
+
 [[example]]
 name = "window_fallthrough"
 path = "examples/ui/window_fallthrough.rs"

examples/README.md

@@ -339,6 +339,7 @@ Example | Description
 [Borders](../examples/ui/borders.rs) | Demonstrates how to create a node with a border
 [Button](../examples/ui/button.rs) | Illustrates creating and updating a button
 [CSS Grid](../examples/ui/grid.rs) | An example for CSS Grid layout
+[Display and Visibility](../examples/ui/display_and_visibility.rs) | Demonstrates how Display and Visibility work in the UI.
 [Flex Layout](../examples/ui/flex_layout.rs) | Demonstrates how the AlignItems and JustifyContent properties can be composed to layout nodes and position text
 [Font Atlas Debug](../examples/ui/font_atlas_debug.rs) | Illustrates how FontAtlases are populated (used to optimize text rendering internally)
 [Overflow](../examples/ui/overflow.rs) | Simple example demonstrating overflow behavior