This is a follow up to the new API introduced in #126.

Summary of Changes

• A new module was added for defining the events_loop. Most event handling code from the old mod.rs has been moved into this module. The NSEventToEvent function has been moved to a private ns_event_to_event method on the EventsLoop itself.
• The global static mut key modifier trackers (SHIFT_PRESSED, ALT_PRESSED, etc) have been removed in favour a Modifiers struct which is owned by the EventsLoop instance.
• The event module was removed in favour of moving the one function that was inside of it (used for converting cocoa key codes to VirtualKeyCodes) to the new events_loop module.
• Window, WindowDelegate, DelegateState and related items have been moved from mod.rs into a new window.rs module.
• The old PollEventsIterator and WaitEventsIterators have been removed in favour of the new EventsLoop::poll_events and EventsLoop::run_forever methods respectively.
• WindowProxy was removed in favour of the new API. Its logic for waking up the waiting event loop was moved to the EventsLoop::interrupt method.
• Add a test that will only compile if the EventsLoop is Send + Sync. This should make it clearer to contributors that the backend that they're working on must be Send + Sync.

Fixes

• Fixes the issue where window resize events would not be emitted until the end of the resize. This PR fixes Resize events not working on OSX #39 by ensuring that the user callback given to either EventsLoop::poll_events or EventsLoop::run_forever can be called by each window delegate's resize callback directly.
• Fixes Example apps segfault on macOS when opening spotlight menu #104, see this comment for explanation.
• Fixes the multiwindow example, closes Multiwindow example is broken on OS X #56.
• Fixes the old behaviour where a Event::Awakened was emitted on every unknown NSEvent. Instead, we now only return Event::Awakened on NSApplicationActivated, a change that was approved for glutin but was never merged due to the changeover of event handling to winit.
• Closes Ensure that OS/X window creation is done in the main thread #31: panic!s if a user attempts to poll for events or create a window on a non-main thread.
• Both poll_events and run_forever will now panic! if a user attempts to call either method on a non-main thread.

unsafe code

Unsafe code was necessary in order to share the user-given callback between each of the window delegate callbacks. A *mut FnMut(Event) is stored behind a Mutex in order to do this. I've explained the behaviour in detailed comments on the UserCallback and its methods. If someone could review the new UserCallback type and its usage, that would be greatly appreciated!

WIP todo:

• Implement interrupted properly. At the moment it causes run_forever to return ASAP after the next event, however it should actually wake-up the "awaiting event" function.
• Ensure that multi-window handling is working properly and that events are correctly emitted for each window (should fix Multiwindow example is broken on OS X #56).
• Work out how to ensure Send + Sync are safely impld for EventsLoop without using unsafe impl. Currently neither are implemented as the EventsLoop's callback field is not Send or Sync:

  the trait `std::marker::Send` is not implemented for `std::ops::FnMut(winit::Event) + 'static
  

  Edit: I think I've fixed this by wrapping the user callback in a UserCallback type and impling Send and Sync for it.
• Ensure that new windows become the "main" window.
• Fix mouse Enter/Leave events. At the moment, Enter is only triggered when the mouse is over the resize bar and Leave triggers when leaving the resize bar 😖. This seems to require setting up tracking areas which seems like a more involved fix. Going to leave this for a future PR.

@tomaka feel free to merge #126 in the meantime if this PR is not yet ready. In that case, I'll just close this and re-open against master.