Proposal - recurring function execution

[thevojacek]

Proposal Proposal is to have a functionality to let user of nanogui schedule his own functions to be executed periodically during main application loop. Execution of those functions depends on set interval in which they should be run.

Reasoning It may be sometimes handy to be able to run some code periodically on main application loop (you can not update UI from another thread directly on macOS for instance) in GUI applications. This PR gives user such ability but without managing GLFW and other stuff themselves and relying only on nanogui, which improves usability of the library.