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/buffer_search.rs

@@ -1,43 +0,0 @@
-use gpui2::{Div, Render, View, VisualContext};
-
-use crate::prelude::*;
-use crate::{h_stack, Icon, IconButton, IconColor, Input};
-
-#[derive(Clone)]
-pub struct BufferSearch {
-    is_replace_open: bool,
-}
-
-impl BufferSearch {
-    pub fn new() -> Self {
-        Self {
-            is_replace_open: false,
-        }
-    }
-
-    fn toggle_replace(&mut self, cx: &mut ViewContext<Self>) {
-        self.is_replace_open = !self.is_replace_open;
-
-        cx.notify();
-    }
-
-    pub fn view(cx: &mut WindowContext) -> View<Self> {
-        cx.build_view(|cx| Self::new())
-    }
-}
-
-impl Render for BufferSearch {
-    type Element = Div<Self>;
-
-    fn render(&mut self, cx: &mut ViewContext<Self>) -> Div<Self> {
-        h_stack().bg(cx.theme().colors().toolbar).p_2().child(
-            h_stack().child(Input::new("Search")).child(
-                IconButton::<Self>::new("replace", Icon::Replace)
-                    .when(self.is_replace_open, |this| this.color(IconColor::Accent))
-                    .on_click(|buffer_search, cx| {
-                        buffer_search.toggle_replace(cx);
-                    }),
-            ),
-        )
-    }
-}