Cocoa: Fix pre-window-creation event processing

Polling the event queue before NSApp had been allowed to finish
launching, in our case by starting our self-terminating run loop,
triggered an assertion inside NSApplication.

This fix, which makes all event processing functions capable of starting
it, makes that assertion less likely.

A more Cocoa-friendly fix would be to finish launching NSApp during
glfwInit and let people annoyed by the menu bar disabled it with
GLFW_COCOA_MENUBAR.  That may not be suitable for 3.3-stable, though.

Fixes #1543.

(cherry picked from commit 6e680500)