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?
```

crates/egui_demo_lib/src/demo/mod.rs

@@ -23,6 +23,7 @@ pub mod paint_bezier;
 pub mod painting;
 pub mod panels;
 pub mod password;
+mod popups;
 pub mod scene;
 pub mod screenshot;
 pub mod scrolling;