Pass a MutableAppContext to Element::layout and ::dispatch_event

We need to mutate the app in these cases to relay layout state, so this just makes things easier. We won't be able to perform layout in parallel but it's questionable whether we'll ever actually do that. If we do, we can revisit.

Co-Authored-By: Antonio Scandurra <me@as-cii.com>
Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>

gpui/src/elements.rs

@@ -29,8 +29,8 @@ pub use uniform_list::*;
 
 use crate::{
     geometry::{rect::RectF, vector::Vector2F},
-    json, AfterLayoutContext, AppContext, DebugContext, Event, EventContext, LayoutContext,
-    PaintContext, SizeConstraint,
+    json, AfterLayoutContext, DebugContext, Event, EventContext, LayoutContext, PaintContext,
+    SizeConstraint,
 };
 use core::panic;
 use json::ToJson;