Add Popup and Tooltip, unifying the previous behaviours (#5713)

This introduces new `Tooltip` and `Popup` structs that unify and extend
the old popups and tooltips.

`Popup` handles the positioning and optionally stores state on whether
the popup is open (for click based popups like `ComboBox`, menus,
context menus).
`Tooltip` is based on `Popup` and handles state of whether the tooltip
should be shown (which turns out to be quite complex to handles all the
edge cases).

Both `Popup` and `Tooltip` can easily be constructed from a `Response`
and then customized via builder methods.

This also introduces `PositionAlign`, for aligning something outside of
a `Rect` (in contrast to `Align2` for aligning inside a `Rect`). But I
don't like the name, any suggestions? Inspired by [mui's tooltip
positioning](https://mui.com/material-ui/react-tooltip/#positioned-tooltips).

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

TODOs:
- [x] Automatic tooltip positioning based on available space
- [x] Review / fix / remove all code TODOs 
- [x] ~Update the helper fns on `Response` to be consistent in naming
and parameters (Some use tooltip, some hover_ui, some take &self, some
take self)~ actually, I think the naming and parameter make sense on
second thought
- [x] Make sure all old code is marked deprecated

For discussion during review:
- the following check in `show_tooltip_for` still necessary?:
```rust
     let is_touch_screen = ctx.input(|i| i.any_touches());
     let allow_placing_below = !is_touch_screen; // There is a finger below. TODO: Needed?
```

examples/popups/src/main.rs

@@ -1,7 +1,7 @@
 #![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::{popup_below_widget, CentralPanel, ComboBox, Id, PopupCloseBehavior};
+use eframe::egui::{CentralPanel, ComboBox, Popup, PopupCloseBehavior};
 
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
@@ -42,23 +42,14 @@ impl eframe::App for MyApp {
 
             ui.label("PopupCloseBehavior::IgnoreClicks 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::IgnoreClicks,
-                |ui| {
+            Popup::menu(&response)
+                .close_behavior(PopupCloseBehavior::IgnoreClicks)
+                .show(|ui| {
                     ui.set_min_width(310.0);
                     ui.label("This popup will be open until you press the button again");
                     ui.checkbox(&mut self.checkbox, "Checkbox");
-                },
-            );
+                });
         });
     }
 }