refactor project structure (start of redoing atomic branch without atomics)

core/src/layout/attr.rs

@@ -0,0 +1,20 @@
+use crate::layout::{Ui, WidgetId, WidgetIdFn, WidgetLike};
+
+pub trait WidgetAttr<W> {
+    type Input;
+    fn run(ui: &mut Ui, id: &WidgetId<W>, input: Self::Input);
+}
+
+pub trait Attrable<W, Tag> {
+    fn attr<A: WidgetAttr<W>>(self, input: A::Input) -> impl WidgetIdFn<W>;
+}
+
+impl<WL: WidgetLike<Tag>, Tag> Attrable<WL::Widget, Tag> for WL {
+    fn attr<A: WidgetAttr<WL::Widget>>(self, input: A::Input) -> impl WidgetIdFn<WL::Widget> {
+        |ui| {
+            let id = self.add(ui);
+            A::run(ui, &id, input);
+            id
+        }
+    }
+}