Add example for click callbacks.

examples/Example11_clickCallback/Example11_clickCallback.ino

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+#include <retroTerm.h> //Include terminal library
+retroTerm terminal; //Create a terminal instance
+uint8_t button1Id = 0; //Record the button ID globally
+uint8_t button2Id = 0; //Record the button ID globally
+uint8_t button3Id = 0; //Record the button ID globally
+uint32_t numberOfClicks[3] = {1,1,1}; //Record the number of clicks
+
+void processClick(uint8_t widgetClicked) {
+  if(widgetClicked == button1Id) {
+    terminal.scroll("Button 1 click " + String(numberOfClicks[0]++)); //Print inside the scroll window
+  } else if(widgetClicked == button2Id) {
+    terminal.scroll("Button 2 click " + String(numberOfClicks[1]++)); //Print inside the scroll window
+  } else if(widgetClicked == button3Id) {
+    terminal.scroll("Button 3 click " + String(numberOfClicks[2]++)); //Print inside the scroll window
+  } else {
+    terminal.scroll("Unknown click"); //Print inside the scroll window
+  }
+}
+
+void setup() {
+  Serial.begin(115200);   //Initialise the Serial stream
+  terminal.begin(Serial); //Initialise the library
+  terminal.eraseScreen(); //Clear the screen
+  terminal.hideCursor(); //Hide the terminal cursor
+  terminal.enableMouse(); //Capture the mouse so it can be used with widgets
+  terminal.setScrollWindow(4,12); //Set up somewhere to show the events without disrupting the button
+  button1Id = terminal.newButton(1, 1, 15, 3, F("Button 1"), COLOUR_RED, OUTER_BOX | BOX_SINGLE_LINE); //Create a green button in a box
+  button2Id = terminal.newButton(16, 1, 15, 3, F("Button 2"), COLOUR_GREEN, OUTER_BOX | BOX_SINGLE_LINE); //Create a green button in a box
+  button3Id = terminal.newButton(31, 1, 15, 3, F("Button 3"), COLOUR_BLUE, OUTER_BOX | BOX_SINGLE_LINE); //Create a green button in a box
+  terminal.widgetShortcutKey(button1Id,f1Pressed); //Assign a shortcut key of F1
+  terminal.widgetShortcutKey(button2Id,f2Pressed); //Assign a shortcut key of F2
+  terminal.widgetShortcutKey(button3Id,f3Pressed); //Assign a shortcut key of F3
+  terminal.showWidget(button1Id); //Make the button visible, all widgets start 'invisible' for later display
+  terminal.showWidget(button2Id); //Make the button visible, all widgets start 'invisible' for later display
+  terminal.showWidget(button3Id); //Make the button visible, all widgets start 'invisible' for later display
+  terminal.setClickCallback(processClick);    //retroTerm callback for click events
+}
+
+void loop() {
+  terminal.houseKeeping();  //You MUST run housekeeping to show/detect any changes or events
+}

examples/Example11_clickCallback/README.md

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+# clickCallback.ino
+
+This example sketch shows how to use the callback function to register clicks from various widgets. As you click the buttons you should get output like this.
+
+![](images/clickCallback.png)