Documentation and symbolic constants for dispatcher actions (#31278)

* Remove unreachable code

`view_last_screen` is never used.

* Remove unused action `user_activity_started`

Nothing listens to this, so it's pointless.

* Symbolic constant for `Action.UserActivity`

* Define symbolic constants for more `Action`s

Constants for some actions that are emitted by `Lifecycle`

test/unit-tests/vector/platform/ElectronPlatform-test.ts

@@ -132,7 +132,7 @@ describe("ElectronPlatform", () => {
         new ElectronPlatform();
         dispatcher.dispatch(
             {
-                action: "client_started",
+                action: Action.ClientStarted,
             },
             true,
         );