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/emath/src/align.rs

@@ -50,6 +50,16 @@ impl Align {
         }
     }
 
+    /// Returns the inverse alignment.
+    /// `Min` becomes `Max`, `Center` stays the same, `Max` becomes `Min`.
+    pub fn flip(self) -> Self {
+        match self {
+            Self::Min => Self::Max,
+            Self::Center => Self::Center,
+            Self::Max => Self::Min,
+        }
+    }
+
     /// Returns a range of given size within a specified range.
     ///
     /// If the requested `size` is bigger than the size of `range`, then the returned
@@ -170,6 +180,24 @@ impl Align2 {
         vec2(self.x().to_sign(), self.y().to_sign())
     }
 
+    /// Flip on the x-axis
+    /// e.g. `TOP_LEFT` -> `TOP_RIGHT`
+    pub fn flip_x(self) -> Self {
+        Self([self.x().flip(), self.y()])
+    }
+
+    /// Flip on the y-axis
+    /// e.g. `TOP_LEFT` -> `BOTTOM_LEFT`
+    pub fn flip_y(self) -> Self {
+        Self([self.x(), self.y().flip()])
+    }
+
+    /// Flip on both axes
+    /// e.g. `TOP_LEFT` -> `BOTTOM_RIGHT`
+    pub fn flip(self) -> Self {
+        Self([self.x().flip(), self.y().flip()])
+    }
+
     /// Used e.g. to anchor a piece of text to a part of the rectangle.
     /// Give a position within the rect, specified by the aligns
     pub fn anchor_rect(self, rect: Rect) -> Rect {