Extract UI elements from storybook into new ui crate (#3008)

This PR extracts the various UI elements from the `storybook` crate into
a new `ui` library crate.

Release Notes:

- N/A

crates/ui/src/components.rs

@@ -0,0 +1,110 @@
+mod facepile;
+mod follow_group;
+mod list_item;
+mod tab;
+
+pub use facepile::*;
+pub use follow_group::*;
+pub use list_item::*;
+pub use tab::*;
+
+use std::marker::PhantomData;
+use std::rc::Rc;
+
+use gpui2::elements::div;
+use gpui2::interactive::Interactive;
+use gpui2::platform::MouseButton;
+use gpui2::style::StyleHelpers;
+use gpui2::{ArcCow, Element, EventContext, IntoElement, ParentElement, ViewContext};
+
+struct ButtonHandlers<V, D> {
+    click: Option<Rc<dyn Fn(&mut V, &D, &mut EventContext<V>)>>,
+}
+
+impl<V, D> Default for ButtonHandlers<V, D> {
+    fn default() -> Self {
+        Self { click: None }
+    }
+}
+
+#[derive(Element)]
+pub struct Button<V: 'static, D: 'static> {
+    handlers: ButtonHandlers<V, D>,
+    label: Option<ArcCow<'static, str>>,
+    icon: Option<ArcCow<'static, str>>,
+    data: Rc<D>,
+    view_type: PhantomData<V>,
+}
+
+// Impl block for buttons without data.
+// See below for an impl block for any button.
+impl<V: 'static> Button<V, ()> {
+    fn new() -> Self {
+        Self {
+            handlers: ButtonHandlers::default(),
+            label: None,
+            icon: None,
+            data: Rc::new(()),
+            view_type: PhantomData,
+        }
+    }
+
+    pub fn data<D: 'static>(self, data: D) -> Button<V, D> {
+        Button {
+            handlers: ButtonHandlers::default(),
+            label: self.label,
+            icon: self.icon,
+            data: Rc::new(data),
+            view_type: PhantomData,
+        }
+    }
+}
+
+// Impl block for button regardless of its data type.
+impl<V: 'static, D: 'static> Button<V, D> {
+    pub fn label(mut self, label: impl Into<ArcCow<'static, str>>) -> Self {
+        self.label = Some(label.into());
+        self
+    }
+
+    pub fn icon(mut self, icon: impl Into<ArcCow<'static, str>>) -> Self {
+        self.icon = Some(icon.into());
+        self
+    }
+
+    pub fn on_click(
+        mut self,
+        handler: impl Fn(&mut V, &D, &mut EventContext<V>) + 'static,
+    ) -> Self {
+        self.handlers.click = Some(Rc::new(handler));
+        self
+    }
+}
+
+pub fn button<V>() -> Button<V, ()> {
+    Button::new()
+}
+
+impl<V: 'static, D: 'static> Button<V, D> {
+    fn render(
+        &mut self,
+        view: &mut V,
+        cx: &mut ViewContext<V>,
+    ) -> impl IntoElement<V> + Interactive<V> {
+        // let colors = &cx.theme::<Theme>().colors;
+
+        let button = div()
+            // .fill(colors.error(0.5))
+            .h_4()
+            .children(self.label.clone());
+
+        if let Some(handler) = self.handlers.click.clone() {
+            let data = self.data.clone();
+            button.on_mouse_down(MouseButton::Left, move |view, event, cx| {
+                handler(view, data.as_ref(), cx)
+            })
+        } else {
+            button
+        }
+    }
+}