Enable more collab UI features (#3496)

* Current Call section of the collab panel
* Improve the collab titlebar
* Add basic UI for following

Following only partially works, but the UI for following is now in
place.

crates/gpui2/src/elements/canvas.rs

@@ -0,0 +1,48 @@
+use crate::{Bounds, Element, IntoElement, Pixels, StyleRefinement, Styled, WindowContext};
+
+pub fn canvas(callback: impl 'static + FnOnce(Bounds<Pixels>, &mut WindowContext)) -> Canvas {
+    Canvas {
+        paint_callback: Box::new(callback),
+        style: Default::default(),
+    }
+}
+
+pub struct Canvas {
+    paint_callback: Box<dyn FnOnce(Bounds<Pixels>, &mut WindowContext)>,
+    style: StyleRefinement,
+}
+
+impl IntoElement for Canvas {
+    type Element = Self;
+
+    fn element_id(&self) -> Option<crate::ElementId> {
+        None
+    }
+
+    fn into_element(self) -> Self::Element {
+        self
+    }
+}
+
+impl Element for Canvas {
+    type State = ();
+
+    fn layout(
+        &mut self,
+        _: Option<Self::State>,
+        cx: &mut WindowContext,
+    ) -> (crate::LayoutId, Self::State) {
+        let layout_id = cx.request_layout(&self.style.clone().into(), []);
+        (layout_id, ())
+    }
+
+    fn paint(self, bounds: Bounds<Pixels>, _: &mut (), cx: &mut WindowContext) {
+        (self.paint_callback)(bounds, cx)
+    }
+}
+
+impl Styled for Canvas {
+    fn style(&mut self) -> &mut crate::StyleRefinement {
+        &mut self.style
+    }
+}

crates/gpui2/src/elements/mod.rs

@@ -1,3 +1,4 @@
+mod canvas;
 mod div;
 mod img;
 mod overlay;
@@ -5,6 +6,7 @@ mod svg;
 mod text;
 mod uniform_list;
 
+pub use canvas::*;
 pub use div::*;
 pub use img::*;
 pub use overlay::*;