Add screen lock mechanism for touch screens (M5 Core2). (#155)

On PWR button double-click, lock or unlock the screen.
This is implemented as a modal message box that just blocks all touch
input.

README.md

@@ -160,6 +160,11 @@ During shutter control:
 * press and hold 'Shutter Lock' to lock shutter open
    * press and hold 'Shutter Lock' again to release
 
+To activate screen lock:
+* double click the PWR button
+   * a message box will open
+* double click the PWR button to unlock
+
 ### Menu Map
 
 * Connect (if connections are saved)

include/FurbleUI.h

@@ -63,6 +63,7 @@ class UI {
     lv_obj_t *batteryIcon;
     lv_obj_t *reconnectIcon;
     lv_obj_t *gpsData;
+    bool screenLocked;
   } status_t;
 
   class Intervalometer {

src/FurbleUI.cpp

@@ -155,6 +155,7 @@ UI::UI(const interval_t &interval) : m_GPS {GPS::getInstance()}, m_Intervalomete
 #else
   m_Status.batteryIcon = addIcon(LV_SYMBOL_BATTERY_3);
 #endif
+  m_Status.screenLocked = false;
 
   m_GPS.setIcon(m_Status.gpsIcon);
 
@@ -193,6 +194,16 @@ UI::UI(const interval_t &interval) : m_GPS {GPS::getInstance()}, m_Intervalomete
         } else {
           lv_obj_add_flag(status->gpsIcon, LV_OBJ_FLAG_HIDDEN);
         }
+
+        static lv_obj_t *lockMsgBox = NULL;
+        if (status->screenLocked && (lockMsgBox == NULL)) {
+          lockMsgBox = lv_msgbox_create(NULL);
+          lv_msgbox_add_title(lockMsgBox, "Screen Locked");
+          lv_msgbox_add_text(lockMsgBox, "Double-click PWR button to unlock.");
+        } else if (!status->screenLocked && (lockMsgBox != NULL)) {
+          lv_msgbox_close_async(lockMsgBox);
+          lockMsgBox = NULL;
+        }
       },
       250, &m_Status);
 
@@ -1780,6 +1791,14 @@ void UI::task(void) {
       // fake PMIC button as actual button
       m_PMICClicked = true;
     }
+
+    // toggle screen lock on power button double click for touch screens
+    if (M5.Touch.isEnabled()) {
+      if (M5.BtnPWR.wasDoubleClicked()) {
+        m_Status.screenLocked = !m_Status.screenLocked;
+      }
+    }
+
     m_Mutex.lock();
     lv_task_handler();
     m_Mutex.unlock();