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/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "popups"
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+version.workspace = true
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+eframe = { workspace = true, features = [
+    "default",
+    "__screenshot", # __screenshot is so we can dump a screenshot using EFRAME_SCREENSHOT_TO
+] }
+env_logger = { version = "0.10", default-features = false, features = [
+    "auto-color",
+    "humantime",
+] }
+
+
+[lints]
+workspace = true

examples/popups/README.md

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

examples/popups/src/main.rs

@@ -0,0 +1,52 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+#![allow(rustdoc::missing_crate_level_docs)] // it's an example
+
+use eframe::egui::*;
+
+fn main() -> Result<(), eframe::Error> {
+    env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
+    let options = eframe::NativeOptions::default();
+
+    eframe::run_native("Popups", options, Box::new(|_| Ok(Box::<MyApp>::default())))
+}
+
+#[derive(Default)]
+struct MyApp {
+    checkbox: bool,
+    number: u8,
+}
+
+impl eframe::App for MyApp {
+    fn update(&mut self, ctx: &eframe::egui::Context, _frame: &mut eframe::Frame) {
+        CentralPanel::default().show(ctx, |ui| {
+            ui.label("PopupCloseBehavior::CloseOnClickAway popup");
+            let response = ui.button("Open");
+            let popup_id = Id::new("popup_id");
+
+            if response.clicked() {
+                ui.memory_mut(|mem| mem.toggle_popup(popup_id));
+            }
+
+            popup_below_widget(
+                ui,
+                popup_id,
+                &response,
+                PopupCloseBehavior::CloseOnClickOutside,
+                |ui| {
+                    ui.set_min_width(300.0);
+                    ui.label("This popup will be open even if you click the checkbox");
+                    ui.checkbox(&mut self.checkbox, "Checkbox");
+                },
+            );
+
+            ui.label("PopupCloseBehavior::CloseOnClick popup");
+            ComboBox::from_label("ComboBox")
+                .selected_text(format!("{}", self.number))
+                .show_ui(ui, |ui| {
+                    for num in 0..10 {
+                        ui.selectable_value(&mut self.number, num, format!("{num}"));
+                    }
+                });
+        });
+    }
+}