Add an EventQueue-based blinky to explain flow control (ARMmbed#115)

To be used in mbed-os-5-docs/docs/api/io/flow_control.md

Tutorials_UsingAPIs/Flow-Control-EventQueue/README.md

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+# Application Flow Control: EventQueue example
+
+An EventQueue Blinky implementation.

Tutorials_UsingAPIs/Flow-Control-EventQueue/main.cpp

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+/*
+ * Copyright (c) 2020 Arm Limited and affiliates.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include "mbed.h"
+
+void flip(DigitalOut &led)
+{
+    led = !led;
+}
+
+int main()
+{
+    DigitalOut led1(LED1);
+    DigitalOut led2(LED2);
+
+    // creates a queue with the default size
+    EventQueue queue;
+
+    // events are simple callbacks
+    queue.call_every(1000, flip, led1);
+    queue.call_every(500, flip, led2);
+
+    // events are executed by the dispatch method
+    queue.dispatch_forever();
+}