* Split the UI overflow implementation so overflow can be set for each axis separately.
* Added new enum `OverflowAxis` with `Hidden` and `Visible` variants.
* Changed `Overflow` to a struct with `x` and `y` fields of type `OverflowAxis`.
* `Overflow` has new methods `visible()` and `hidden()` that replace its previous `Hidden` and `Visible` variants.
* Added `Overflow` helper methods `x_hidden()` and `y_hidden()` that return a new `Overflow` with the given axis hidden.
* Added register type for `OverflowAxis`.
* Changed the `ui` example to use `Overflow::y_hidden()` instead of `Overflow::Hidden`.
* Modified `update_clipping` so it calculates clipping for each axis separately. If a node is only clipped on one axis, the other axis is given -/+ `f32::INFINITY` clipping bounds.

* Added `overflow_debug` example demonstrating how to use the `overflow`.

* Added `is_visible` methods to `Overflow` and `OverflowAxis`
* Replaced the second `match` statement in `update_clipping` with an `if` statement.

* Cargo fmt
* Overflow example clips a bevy logo image instead of a rectangle.

* Renamed `OverflowAxis::Hidden` to `OverflowAxis::Clip`
* Replaced `hide`/`hidden` with `clip in the names of the functions belonging to `Overflow` and `OverflowAxis`.
* Fixed errors in the examples.

…/bevy into split-overflow-by-axes