Improve documentation about concurrency and *Gui.Execute()

[jroimartin]

Explain how concurrency works in gocui under the hood. For instance, write a bit about channels for keyboard/mouse and user events and put more emphasis on how to modify views from goroutines.

Relates to #77

[gbitten]

When I used gocui recently, I made the mistake of calling SetKeybinding from a Layout method. That made my system works weird, binding the same key multiple times. Saying that, maybe the best way for the documentation is to define the best practices or patterns to use gocui's methods in general.

[awnumar]

I agree with @gbitten, but what should be considered is that good practices can be found in the examples directory. However, they only teach people how they should use something, not how it works or why that way. The documentation is quite obscure in that respect.

[jroimartin]

Function renamed to *Gui.Update() + more docs in 6564cfcacb01db61e3a4e983405a598db3e91982.