Key element outline items by assembly

Introduce an assembly serial number, which increments when we load a new
assembly. Include it in each element outline item's diff key.

app-proto/src/main.rs

@@ -15,6 +15,7 @@ use outline::Outline;
 #[derive(Clone)]
 struct AppState {
     assembly: Assembly,
+    assembly_serial: Signal<u32>,
     selection: Signal<FxHashSet<ElementKey>>
 }
 
@@ -22,6 +23,7 @@ impl AppState {
     fn new() -> AppState {
         AppState {
             assembly: Assembly::new(),
+            assembly_serial: create_signal(1),
             selection: create_signal(FxHashSet::default())
         }
     }