Re-visit ImGui.Input

[wrestledBearOnce]

Currently the ImGuiInputDriver is very bare bone and not production ready.

Re-iterate and implement with different keyboard layouts (and languages) and input vs. UTF-8 text output in mind.

@griestopf it would be nice if we could meet and discuss how to achieve a clean and good implementation. We could use something like this: https://stackoverflow.com/a/47409469 (GetEncoding()) alternatively Char.Parse(char, CultureInfo).

[griestopf]

Also look at #651
Also look at how Z/Y are (mis-) handled on English/German keyboard layout (Input.Keyboard.IsKeydown('Z') returns the state for the Y key and vice versa).

[RedImp1470]