Add goroutine6

[rcalixte]

Ported from:
https://github.com/therecipe/examples/blob/master/advanced/widgets/goroutine/main.go

The labels and the goroutines sometimes freeze until the window focus is changed or the window is moved. I tried different time values but wasn't able to have a 100% consistent execution. It works well enough for an example hopefully.

[mappu]

This is a really good example, and I think the stuttering shows that goroutines must currently be handled slightly differently in MIQT. It is possible to make this perfectly smooth, but MIQT does not yet have an equivalent to therecipe/qt's label.UpdateTextFromGoroutine method.

I'd like to fix the bug and make it smooth before merging it. This is also related to #72.

Probably the fix is either

• (Fix Go-side): the goroutine should push to a channel, and the main thread should pull channel events (but this needs a way to integrate with the blocking Exec() call); or else
• (Fix Qt-side): emit a signal with a callback parameter, which can always be caught and executed on the main event loop