This is the initial implementation of glfwDragWindow, with support for
X11. The function glfwDragWindow requires only the target window to be
dragged. To make the function easier and more portable, the position of
the window and of the cursor are grabbed internally, so the end-user do
not need to pass them manually.
The example 'simple.c' was updated to include this functionality when
clicking on the client area of the window.
Replaced the GLFW_SHOULD_CLOSE window parameter with the
glfwWindowShouldClose and glfwSetWindowShouldClose functions, allowing
the setting of the close flag from any point in the program.
