Add glfwShowWindow, glfwHideWindow

Add glfwShowWindow and glfwHideWindow functions to allow explicit
control over show/hide window.

Remove platform specific show window code from _glfwPlatformCreateWindow
but call glfwShowWindow from glfwCreateWindow to avoid breaking things
(for now).

src/internal.h

@@ -302,6 +302,8 @@ void _glfwPlatformSetWindowSize(_GLFWwindow* window, int width, int height);
 void _glfwPlatformSetWindowPos(_GLFWwindow* window, int x, int y);
 void _glfwPlatformIconifyWindow(_GLFWwindow* window);
 void _glfwPlatformRestoreWindow(_GLFWwindow* window);
+void _glfwPlatformShowWindow(_GLFWwindow* window);
+void _glfwPlatformHideWindow(_GLFWwindow* window);
 
 // Event management
 void _glfwPlatformPollEvents(void);