feat(custom): support mouse wheel on slider

docs/modules/Custom.md

@@ -77,7 +77,7 @@ Note that `on_change` will provide the **floating point** value as an argument.
 If your input program requires an integer, you will need to round it.
 
 | Name          | Type                                               | Default      | Description                                                                                                                     |
-|---------------|----------------------------------------------------|--------------|------------------------------------------------------------------------------|
+|---------------|----------------------------------------------------|--------------|---------------------------------------------------------------------------------------------------------------------------------|
 | `src`         | `image`                                            | `null`       | Image source. See [here](images) for information on images.                                                                     |
 | `size`        | `integer`                                          | `null`       | Width/height of the image. Aspect ratio is preserved.                                                                           |
 | `orientation` | `horizontal` or `vertical` (shorthand: `h` or `v`) | `horizontal` | Orientation of the slider.                                                                                                      |
@@ -85,6 +85,7 @@ If your input program requires an integer, you will need to round it.
 | `on_change`   | `string [command]`                                 | `null`       | Command to execute when the slider changes. More on this [below](#commands).                                                    | 
 | `min`         | `float`                                            | `0`          | Minimum slider value.                                                                                                           | 
 | `max`         | `float`                                            | `100`        | Maximum slider value.                                                                                                           | 
+| `step`        | `float`                                            | -            | The increment to change when scrolling with the mouse wheel. If left blank, will use the default determined by the environment. | 
 | `length`      | `integer`                                          | `null`       | Slider length. GTK will automatically size if left unset.                                                                       |
 
 The example slider widget below shows a volume control for MPC, 

src/modules/custom/slider.rs

@@ -7,6 +7,7 @@ use gtk::prelude::*;
 use gtk::Scale;
 use serde::Deserialize;
 use std::cell::Cell;
+use std::ops::Neg;
 use tokio::spawn;
 use tracing::error;
 
@@ -21,6 +22,7 @@ pub struct SliderWidget {
     min: f64,
     #[serde(default = "default_max")]
     max: f64,
+    step: Option<f64>,
     length: Option<i32>,
 }
 
@@ -53,11 +55,26 @@ impl CustomWidget for SliderWidget {
         if let Some(on_change) = self.on_change {
             let min = self.min;
             let max = self.max;
+            let step = self.step;
             let tx = context.tx.clone();
 
             // GTK will spam the same value over and over
             let prev_value = Cell::new(scale.value());
 
+            scale.connect_scroll_event(move |scale, event| {
+                let value = scale.value();
+                let delta = event.delta().1.neg();
+
+                let delta = match (step, delta.is_sign_positive()) {
+                    (Some(step), true) => step,
+                    (Some(step), false) => -step,
+                    (None, _) => delta,
+                };
+
+                scale.set_value(value + delta);
+                Inhibit(false)
+            });
+
             scale.connect_change_value(move |scale, _, val| {
                 // GTK will send values outside min/max range
                 let val = val.clamp(min, max);