ui2: Reorganize components (#3228)

This PR reorganizes the components in the `ui2` crate.

The distinction between "elements" and "components" is now gone, with
all of the reusable components living under `components/`.

The components that we built while prototyping but will eventually live
in other crates currently reside in the `to_extract/` module.

Release Notes:

- N/A

crates/ui2/src/components/tool_divider.rs

@@ -0,0 +1,14 @@
+use crate::prelude::*;
+
+#[derive(Component)]
+pub struct ToolDivider;
+
+impl ToolDivider {
+    pub fn new() -> Self {
+        Self
+    }
+
+    fn render<V: 'static>(self, _view: &mut V, cx: &mut ViewContext<V>) -> impl Component<V> {
+        div().w_px().h_3().bg(cx.theme().colors().border)
+    }
+}