Added methods to change NexButton background picture

NexButton.cpp

@@ -41,3 +41,25 @@ bool NexButton::setText(const char *buffer)
     return recvRetCommandFinished();    
 }
 
+bool NexButton::getPIC(uint32_t * number)
+{
+    String cmd = String("get ");
+    cmd += getObjName();
+    cmd += ".pic";
+    sendCommand(cmd.c_str());
+    return recvRetNumber(number);
+}
+
+bool NexButton::setPIC(uint32_t number)
+{
+    char buf[10] = {0};
+    String cmd;
+
+    utoa(number, buf, 10);
+    cmd += getObjName();
+    cmd += ".pic=";
+    cmd += buf;
+
+    sendCommand(cmd.c_str());
+    return recvRetCommandFinished();
+}

NexButton.h

@@ -56,7 +56,26 @@ class NexButton: public NexTouch
      * @param buffer - text buffer terminated with '\0'. 
      * @return true if success, false for failure. 
      */
-    bool setText(const char *buffer);    
+    bool setText(const char *buffer); 
+
+    /**
+     * Sets the background picture property of the button.
+     * 
+     * @param number - Pointer to buffer that will store the ID of the image
+     * resource on Nextion Editor.
+     * 
+     * @return true if command succeeded, false otherwise
+     */
+    bool getPic(uint32_t number);
+
+    /**
+     * Sets the background picture property of the button.
+     * 
+     * @param number - ID of the image resource on Nextion Editor.
+     * 
+     * @return true if command succeeded, false otherwise
+     */
+    bool setPic(uint32_t number);
 };
 /**
  * @}