Restructure monitor enumeration

This way is both kinder on event-based enumeration and less work to
unwind allocations for when properly implementing GLFW_OUT_OF_MEMORY.

src/cocoa_platform.h

@@ -158,6 +158,7 @@ typedef struct _GLFWtimeNS
 
 void _glfwInitTimerNS(void);
 
+void _glfwPollMonitorsNS(void);
 GLFWbool _glfwSetVideoModeNS(_GLFWmonitor* monitor, const GLFWvidmode* desired);
 void _glfwRestoreVideoModeNS(_GLFWmonitor* monitor);