Add PopupCloseBehavior (#4636)

This PR adds `PopupCloseBehavior` to improve state of the
<https://github.com/emilk/egui/issues/4607>

`PopupCloseBehavior` determines when popup will be closed.
- `CloseOnClick` popup will be closed if the click happens anywhere even
in the popup's body
- `CloseOnClickAway` popup will be closed if the click happens somewhere
else but in the popup's body.

It also adds a test in the demo app which contains several popups
examples.

---

My ideas about <https://github.com/emilk/egui/issues/4607> is to make
every tooltip and popup a menu. So it will provide more control over
popups and tooltips (you will be able to close a popup by calling
something similar to the `ui.close_menu` if you need to). You won't need
to manually handle it's opening. And also will allow to have multiple
popups opened. That means you can have a popup inside a popup. And it
will also lead to the easier creation of the popups. (should we create a
tracking issue to track changes because to me it seems like a huge
amount of changes to be done?)

---

- Improvements on <https://github.com/emilk/egui/issues/4607>

examples/popups/README.md

@@ -0,0 +1,5 @@
+Example of how to use menus, popups, context menus and tooltips.
+
+```sh
+cargo run -p popups
+```