⚠️ Improved menu based on egui::Popup (#5716)

Continuation of #5713

**Silently breaking changes:**
- Menus now close on click by default, this is configurable via
`PopupCloseBehavior`

**Additional additions:**
- `Button::right_text`
- `StyleModifier`

This is a rewrite of the egui menus, with the following goals:
- submenu buttons should work everywhere, in a popup, context menu,
area, in some random Ui
- remove the menu state from Ui
- should work just like the previous menu
- ~proper support for keyboard navigation~
  - It's better now but requires further work to be perfect
- support `PopupCloseBehavior`

* Closes #4607 
* [x] I have followed the instructions in the PR template

crates/egui_demo_lib/src/demo/mod.rs

@@ -7,7 +7,6 @@
 pub mod about;
 pub mod code_editor;
 pub mod code_example;
-pub mod context_menu;
 pub mod dancing_strings;
 pub mod demo_app_windows;
 pub mod drag_and_drop;