feat(element): parameter *const App.Model

The renderer will provide `resize`, `reposition` and `minSize` (for
the `Scrollable` `Element`) with a read-only pointer to the model of
the application (similar to how it is already done for `handle` and
`content`). Every interface function now has the same data that it can
each use for implementing its corresponding task based on local and
shared variables through the element instance and model pointer.

examples/styles/palette.zig

@@ -89,8 +89,8 @@ pub fn main() !void {
             else => {},
         }
 
-        container.resize(try renderer.resize());
-        container.reposition(.{});
+        container.resize(&app.model, try renderer.resize());
+        container.reposition(&app.model, .{});
         try renderer.render(@TypeOf(container), &container, App.Model, &app.model);
         try renderer.flush();
     }