panic: send on closed channel about gxui HOT 5 CLOSED

Possibly related to #48

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I looked at #48 first, but the error messages are pretty different.

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I think this is a different from #48. Running with the race detector the relevant info is...

WARNING: DATA RACE
Read by goroutine 47:
runtime.chansend()
/build/go/src/go-1.4.2/src/runtime/chan.go:83 +0x0
github.com/google/gxui/themes/dark.func·011()
/home/paul/src/gosrc/src/github.com/google/gxui/themes/dark/progress_bar.go:39 +0x312

Previous write by main goroutine:
runtime.closechan()
/build/go/src/go-1.4.2/src/runtime/chan.go:257 +0x0
github.com/google/gxui/drivers/gl.func·013()
/home/paul/src/gosrc/src/github.com/google/gxui/drivers/gl/driver.go:115 +0x1e3
github.com/google/gxui/drivers/gl.(*Driver).run()
/home/paul/src/gosrc/src/github.com/google/gxui/drivers/gl/driver.go:85 +0x90
github.com/google/gxui/drivers/gl.StartDriver()
/home/paul/src/gosrc/src/github.com/google/gxui/drivers/gl/driver.go:47 +0x23d
main.main()
/home/paul/src/gosrc/src/github.com/google/gxui/samples/progress_bar/main.go:50 +0x37

Goroutine 47 (running) created at:
time.goFunc()
/build/go/src/go-1.4.2/src/time/sleep.go:129 +0x59

The problem is that there is no communication mechanism to notify the contols of impending termination. The driver attempts to flush any outstanding events prior to closing the event channels, but it's useless because it never told the controls to stop sending events.

There probably needs to be some sort of onTerminate or onDestroy event implemented by any object in the system. As parent objects (containers) are destroyed they would first propagate the signal to their children before then doing any necessary cleanup themselves. After everything has had a chance to clean up then the flush and channel closing can be done.

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This should now be fixed. Please reopen if not.

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Thanks!

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