ui crate docs & spring cleaning (#18768)

Similar to https://github.com/zed-industries/zed/pull/18690 &
https://github.com/zed-industries/zed/pull/18695, this PR enables
required docs for `ui` and does some cleanup.

Changes:
- Enables the `deny(missing_docs)` crate-wide.
- Adds `allow(missing_docs)` on many modules until folks pick them up to
document them
- Documents some modules (all in `ui/src/styles`)
- Crate root-level organization: Traits move to `traits`, other misc
organization
- Cleaned out a bunch of unused code.

Note: I'd like to remove `utils/format_distance` but the assistant panel
uses it. To move it over to use the `time_format` crate we may need to
update it to use `time` instead of `chrono`. Needs more investigation.

Release Notes:

- N/A

crates/ui/src/utils/format_distance.rs

@@ -1,3 +1,6 @@
+// This won't be documented further as it is intended to be removed, or merged with the `time_format` crate.
+#![allow(missing_docs)]
+
 use chrono::{DateTime, Local, NaiveDateTime};
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]

crates/ui/src/utils/with_rem_size.rs

@@ -0,0 +1,83 @@
+use gpui::{
+    div, AnyElement, Bounds, Div, DivFrameState, Element, ElementId, GlobalElementId, Hitbox,
+    IntoElement, LayoutId, ParentElement, Pixels, StyleRefinement, Styled, WindowContext,
+};
+
+/// An element that sets a particular rem size for its children.
+pub struct WithRemSize {
+    div: Div,
+    rem_size: Pixels,
+}
+
+impl WithRemSize {
+    /// Create a new [WithRemSize] element, which sets a
+    /// particular rem size for its children.
+    pub fn new(rem_size: impl Into<Pixels>) -> Self {
+        Self {
+            div: div(),
+            rem_size: rem_size.into(),
+        }
+    }
+}
+
+impl Styled for WithRemSize {
+    fn style(&mut self) -> &mut StyleRefinement {
+        self.div.style()
+    }
+}
+
+impl ParentElement for WithRemSize {
+    fn extend(&mut self, elements: impl IntoIterator<Item = AnyElement>) {
+        self.div.extend(elements)
+    }
+}
+
+impl Element for WithRemSize {
+    type RequestLayoutState = DivFrameState;
+    type PrepaintState = Option<Hitbox>;
+
+    fn id(&self) -> Option<ElementId> {
+        self.div.id()
+    }
+
+    fn request_layout(
+        &mut self,
+        id: Option<&GlobalElementId>,
+        cx: &mut WindowContext,
+    ) -> (LayoutId, Self::RequestLayoutState) {
+        cx.with_rem_size(Some(self.rem_size), |cx| self.div.request_layout(id, cx))
+    }
+
+    fn prepaint(
+        &mut self,
+        id: Option<&GlobalElementId>,
+        bounds: Bounds<Pixels>,
+        request_layout: &mut Self::RequestLayoutState,
+        cx: &mut WindowContext,
+    ) -> Self::PrepaintState {
+        cx.with_rem_size(Some(self.rem_size), |cx| {
+            self.div.prepaint(id, bounds, request_layout, cx)
+        })
+    }
+
+    fn paint(
+        &mut self,
+        id: Option<&GlobalElementId>,
+        bounds: Bounds<Pixels>,
+        request_layout: &mut Self::RequestLayoutState,
+        prepaint: &mut Self::PrepaintState,
+        cx: &mut WindowContext,
+    ) {
+        cx.with_rem_size(Some(self.rem_size), |cx| {
+            self.div.paint(id, bounds, request_layout, prepaint, cx)
+        })
+    }
+}
+
+impl IntoElement for WithRemSize {
+    type Element = Self;
+
+    fn into_element(self) -> Self::Element {
+        self
+    }
+}