Fixed creating contexts without native windows.

To create a context for the GTK GLWidget, you need to be able to
specify the exact NSView you want the OpenGL context to apply to. Also,
you don’t want to initialise the NSApplication in this situation, which
the CocoaContext did before (unintentionally).
Added Quit menu item + shortcut (Cmd+Q).
[Mac] Cmd-Q should raise Closing events

It should also be cancelable.
Merge branch 'pr/105' into develop