Add `glfwPostEmptyEvent` function definition for CFFI

[borodust]

Bump glfw minor version

glfwPostEmptyEvent function is particularly useful when used in pair with glfwWaitEvents to wake up waiting event loop. E.g. when closing window by some internal logic (while setting glfwSetWindowShouldClose) instead of waiting for any other random glfw event to come, one can just call glfwPostEmptyEvent and glfwWaitEvents will return asap.

This function was introduced in glfw 3.1, hence the minor version bump.

[AlexCharlton]

Looks good. Thanks @borodust !