Start assembly serial number at zero

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/add_remove.rs

@@ -148,6 +148,9 @@ pub fn AddRemove() -> View {
             assembly.elements_by_id.update(|elts_by_id| elts_by_id.clear());
             state.selection.update(|sel| sel.clear());
             
+            // increment assembly serial number
+            state.assembly_serial.set_fn_silent(|serial| serial.wrapping_add(1));
+            
             // load assembly
             match name.as_str() {
                 "general" => load_gen_assemb(assembly),

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(0),
             selection: create_signal(FxHashSet::default())
         }
     }

app-proto/src/outline.rs

@@ -180,11 +180,15 @@ pub fn Outline() -> View {
     
     // list the elements alphabetically by ID
     let element_list = state.assembly.elements.map(
-        |elts| elts
-            .clone()
-            .into_iter()
-            .sorted_by_key(|(_, elt)| elt.id.clone())
-            .collect()
+        move |elts| {
+            let asm_serial = state.assembly_serial.get_untracked();
+            elts
+                .clone()
+                .into_iter()
+                .sorted_by_key(|(_, elt)| elt.id.clone())
+                .map(|(key, elt)| (asm_serial, key, elt))
+                .collect()
+        }
     );
     
     view! {
@@ -197,10 +201,10 @@ pub fn Outline() -> View {
         ) {
             Keyed(
                 list=element_list,
-                view=|(key, elt)| view! {
+                view=|(_, key, elt)| view! {
                     ElementOutlineItem(key=key, element=elt)
                 },
-                key=|(key, _)| key.clone()
+                key=|(asm_serial, key, _)| (asm_serial.clone(), key.clone())
             )
         }
     }