Update attention request on X11 and update example

The macro ```_NET_WM_STATE_ADD``` is passed to
```xev.xclient.data.l[0]```, instead of a constant `1`.

The example ```simple.c``` was update to make a attention request every
10 seconds.

examples/simple.c

@@ -79,6 +79,7 @@ int main(void)
     GLFWwindow* window;
     GLuint vertex_buffer, vertex_shader, fragment_shader, program;
     GLint mvp_location, vpos_location, vcol_location;
+    double start;
 
     glfwSetErrorCallback(error_callback);
 
@@ -131,12 +132,18 @@ int main(void)
     glVertexAttribPointer(vcol_location, 3, GL_FLOAT, GL_FALSE,
                           sizeof(vertices[0]), (void*) (sizeof(float) * 2));
 
+    start = glfwGetTime();
     while (!glfwWindowShouldClose(window))
     {
         float ratio;
         int width, height;
         mat4x4 m, p, mvp;
 
+        if (glfwGetTime() - start > 10.0) {
+            glfwRequestWindowAttention(window);
+            start = glfwGetTime();
+        }
+
         glfwGetFramebufferSize(window, &width, &height);
         ratio = width / (float) height;
 

src/null_window.c

@@ -172,6 +172,11 @@ void _glfwPlatformShowWindow(_GLFWwindow* window)
 {
 }
 
+
+void _glfwPlatformRequestWindowAttention(_GLFWwindow* window)
+{
+}
+
 void _glfwPlatformUnhideWindow(_GLFWwindow* window)
 {
 }

src/x11_window.c

@@ -2084,7 +2084,7 @@ void _glfwPlatformRequestWindowAttention(_GLFWwindow* window)
     xev.xclient.window = window->x11.handle;
     xev.xclient.message_type = wm_state;
     xev.xclient.format = 32;
-    xev.xclient.data.l[0] = 1; /* _NET_WM_STATE_ADD */
+    xev.xclient.data.l[0] = _NET_WM_STATE_ADD;
     xev.xclient.data.l[1] = wm_attention;
 
     XSendEvent(_glfw.x11.display, DefaultRootWindow(_glfw.x11.display), False, SubstructureRedirectMask | SubstructureNotifyMask, &xev);