[GUI] Add ti.GUI.EXIT event to close a window

[archibate]

Related issue = #1118

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@xumingkuan Could you do me a favor by testing this sample code:

gui = ti.GUI("Hello")
while gui.running:
   gui.show()

print('GUI closed')

when you have time? Many thanks :)

@k-ye Would you mind to add this feature to cocoa following the win32&x11 as example itpr / iapr? No rush at all, any time before v0.6.8 is ok :)

[k-ye]

Would you mind to add this feature to cocoa following the win32&x11 as example itpr / iapr?

Of course :) (Unless there's someone on a Mac that's interested in this task?)

[xumingkuan]

I can't close the window :(

Hitting the "close" button has no effect.

[archibate]

I can't close the window :(

Hitting the "close" button has no effect.

Sorry! It should works now.

[xumingkuan]

I can't close the window :(
Hitting the "close" button has no effect.

Sorry! It should works now.

Interesting... GUI closed is printed after I hit the button, but the GUI is not closed.

[archibate]

How about this? DestroyWindow?

[xumingkuan]

Same behavior after the new commits...

GUI closed is printed and the window becomes "no response" after I hit the close button.

[xumingkuan]

It works!!!

[archibate]

Great! I'll merge this for now once @yuanming-hu approves me, and please add a Mac version before Jun 7 we release v0.6.8 if someone feel interested :)

[xumingkuan]

LGTM in general. Thanks!

[k-ye]

Great!