Support for additions in 3.1

[schell]

These additions were added in 3.1 and they need to make it into the bindings:

• Added GLFWcursor custom system cursor handle
• Added glfwCreateCursor, glfwCreateStandardCursor, glfwDestroyCursor and glfwSetCursor for managing system cursor images
• Added GLFWimage struct for passing 32-bit RGBA images
• Added monitor and adapter identifier access to native API
• Added glfwSetDropCallback and GLFWdropfun for receiving dropped files
• Added glfwPostEmptyEvent for allowing secondary threads to cause glfwWaitEvents to return
• Added empty test program for verifying posting of empty events
• Added glfwSetCharModsCallback for receiving character events with modifiers
• Added glfwGetWindowFrameSize for retrieving the size of the frame around the client area of a window
• Added GLFW_AUTO_ICONIFY for controlling whether full screen windows automatically iconify (and restore the previous video mode) on focus loss
• Added GLFW_DONT_CARE for indicating that any value is acceptable
• Added GLFW_DOUBLEBUFFER for controlling whether to use double buffering
• Added GLFW_CONTEXT_RELEASE_BEHAVIOR and values GLFW_ANY_RELEASE_BEHAVIOR, GLFW_RELEASE_BEHAVIOR_FLUSH and GLFW_RELEASE_BEHAVIOR_NONE for GL_KHR_context_flush_control support
• Added GLFW_INCLUDE_ES31 for including the OpenGL ES 3.1 header
• Added GLFW_FLOATING for creating always-on-top windowed mode windows
• Added GLFW_FOCUSED window hint for controlling initial input focus

[schell]

Work is being tracked in the branch additions-3.1

[schell]

http://www.glfw.org/docs/latest/news.html#news_31

[Mokosha]

This is now being tracked in #44.