dashboard-windows: match the mailbox class by undecorated methods

WebSocketSend is a MobX @action.bound method, a getter on the prototype in
the page (a plain method only in the offline checker's sandbox), so the
Mailbox signature never matched live. Use SendMessage,
SendMessageAndWaitForResponse and RegisterHandler instead; note the pitfall
at protoMethods in finders.js.

modules/lib/finders.js

@@ -27,7 +27,10 @@
 //       "props": { "k": value }     data properties (own or inherited) equal to
 //                                   the given JSON primitives; getters never run,
 //       "has": ["k", ...]           properties present (`in`, incl. getters),
-//       "protoMethods": ["m", ...]  (class) methods on its prototype chain,
+//       "protoMethods": ["m", ...]  (class) methods on its prototype chain
+//                                   (plain methods only: decorated ones, e.g.
+//                                   MobX @action.bound, are getters in the
+//                                   page, though methods offline),
 //       "methods": ["m", ...]       (instance) methods on its prototype chain,
 //       "getters": ["g", ...]       getters on its prototype chain }
 //   Fields that only scripts/check-signatures.mjs uses: "expects" (strings the

modules/lib/signatures.json

@@ -106,7 +106,7 @@
         "mailbox": {
           "module": { "includes": ["\"mailbox_send \"", "WebSocketSend(", "\"vrcompositor_systemlayer\""] },
           "exports": {
-            "Mailbox": { "type": "class", "protoMethods": ["SendMessage", "WebSocketSend"] }
+            "Mailbox": { "type": "class", "protoMethods": ["SendMessage", "SendMessageAndWaitForResponse", "RegisterHandler"] }
           }
         },
         "sceneGraph": {