Add support for unicode characters/text

[elgopher]

At the moment keyboard package can be used for checking the state of the keyboard. This is enough for most games. But some games have text input fields and support entering diacritic characters. They take also into the consideration the layout of the keyboard.

The API might be a new package which will provide some kind of buffer:

text := textinput.New(window)
text.Update()    // standard Update() method executed each frame
text.Text()         // text entered so far in UTF-8 (normal Go string)
text.Clear()       // clear the buffer