scratch like broadcast api (#1358)

* scratch like broadcast api

* always register handler

* adding docs

* added icon

* updated block name

* adding DAL support "radio.raiseEvent"

* typo

* implemented using radiobus

* adding docs

* adding raise event block

* updated shims

* updated pxt

sim/visuals/microbit.ts

@@ -854,6 +854,9 @@ path.sim-board {
                 switch (msg.type || "") {
                     case "serial": this.flashSystemLed(); break;
                     case "radiopacket": this.flashAntenna(); break;
+                    case "eventbus":
+                        if ((<pxsim.SimulatorEventBusMessage>msg).id == DAL.MES_BROADCAST_GENERAL_ID)
+                            this.flashAntenna(); break;
                 }
             }
             let tiltDecayer = 0;