Add scrubbing / dragging to number_input widget. (#24636)

# Objective

Part of #19236 

This PR is a near-total rewrite of the feathers number input widget.
This version adds the ability to edit the value by dragging,
behaviorally similar to the widgets in Blender.

## Features

* Click to edit textually
* Drag to adjust
* Adaptive drag speed heuristic
* Conditional slider bar (only appears if soft limits are specified)
* Soft and hard limits (ref. [Blender
documentation](https://docs.blender.org/manual/en/latest/interface/controls/buttons/fields.html#number-fields))
* Precision and step options
* Cursor shape changing
* Support for different number formats: f32, f64, i32, i64

## Backwards compatibility

The method for programmatically updating the edited value has changed to
be more consistent with other widgets - instead of sending an event, you
now insert a `NumberInputValue` component with the new value.

## Testing

- extensive manual testing via a new example, `feathers_number_input`.

## Additional Information

Things left to do:

* Visible increment / decrement arrows (only for fields with no soft
limit)
* Support for unit conversions and units display (meters, centimeters,
degrees, etc.)
* The theme colors are not final; there's some debate about whether we
will need to implement multiple thematic variations of this widget in
order to maintain sufficient contrast against different backgrounds.
* Accessibility support needs work.
* The example has some placeholder labels that we supposed to mirror the
input value, but that has not been finished.
* Does not enable drag capturing in CursorOption because bevy_picking
doesn't work with captured pointers.

Note: because of the complexity of this widget, it does not use the
"slider" headless widget, and instead is based on the `EditableText`
component, with slider-like behaviors layered on top.

Also, this widget pretty much subsumes all of the functionality of the
feathers slider, making the latter unnecessary. The only advantage of
`FeathersSlider` is that, since it only supports `f32`, it has a simpler
API.

---------

Co-authored-by: ickshonpe <david.curthoys@googlemail.com>
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

Author: 3 people

Cargo.toml

@@ -5497,6 +5497,18 @@ description = "Gallery of Feathers Widgets"
 category = "UI (User Interface)"
 wasm = true
 
+[[example]]
+name = "feathers_number_input"
+path = "examples/ui/widgets/feathers_number_input.rs"
+doc-scrape-examples = true
+required-features = ["bevy_feathers"]
+
+[package.metadata.example.feathers_number_input]
+name = "Feathers Number Input"
+description = "Feathers Number Input Options"
+category = "UI (User Interface)"
+wasm = true
+
 [[example]]
 name = "render_depth_to_texture"
 path = "examples/shader_advanced/render_depth_to_texture.rs"