Add support for optional icon to Button (#3479)

This PR extends `Button` with support for an optional icon to be
displayed next to the label.

As part of this, the functionality for displaying an icon within a
button has been factored out into an internal `ButtonIcon` component.
`ButtonIcon` is now used by both `IconButton` and `Button` to
encapsulate the concerns of an icon that is rendered within a button.

Release Notes:

- N/A

crates/ui2/src/components/button/button_icon.rs

@@ -0,0 +1,84 @@
+use crate::{prelude::*, Icon, IconElement, IconSize};
+
+/// An icon that appears within a button.
+///
+/// Can be used as either an icon alongside a label, like in [`Button`](crate::Button),
+/// or as a standalone icon, like in [`IconButton`](crate::IconButton).
+#[derive(IntoElement)]
+pub(super) struct ButtonIcon {
+    icon: Icon,
+    size: IconSize,
+    color: Color,
+    disabled: bool,
+    selected: bool,
+    selected_icon: Option<Icon>,
+}
+
+impl ButtonIcon {
+    pub fn new(icon: Icon) -> Self {
+        Self {
+            icon,
+            size: IconSize::default(),
+            color: Color::default(),
+            disabled: false,
+            selected: false,
+            selected_icon: None,
+        }
+    }
+
+    pub fn size(mut self, size: impl Into<Option<IconSize>>) -> Self {
+        if let Some(size) = size.into() {
+            self.size = size;
+        }
+
+        self
+    }
+
+    pub fn color(mut self, color: impl Into<Option<Color>>) -> Self {
+        if let Some(color) = color.into() {
+            self.color = color;
+        }
+
+        self
+    }
+
+    pub fn selected_icon(mut self, icon: impl Into<Option<Icon>>) -> Self {
+        self.selected_icon = icon.into();
+        self
+    }
+}
+
+impl Disableable for ButtonIcon {
+    fn disabled(mut self, disabled: bool) -> Self {
+        self.disabled = disabled;
+        self
+    }
+}
+
+impl Selectable for ButtonIcon {
+    fn selected(mut self, selected: bool) -> Self {
+        self.selected = selected;
+        self
+    }
+}
+
+impl RenderOnce for ButtonIcon {
+    type Rendered = IconElement;
+
+    fn render(self, _cx: &mut WindowContext) -> Self::Rendered {
+        let icon = self
+            .selected_icon
+            .filter(|_| self.selected)
+            .unwrap_or(self.icon);
+
+        let icon_color = if self.disabled {
+            Color::Disabled
+        } else if self.selected {
+            Color::Selected
+        } else {
+            self.color
+        };
+
+        IconElement::new(icon).size(self.size).color(icon_color)
+    }
+}