[Lua] Input binding renderer

OpenMW needs a keybind as one of the core renderers for the settings window. It will eventually need to support other types of input like touch and controllers, but at bare minimum we need to support keyboard.

It is possible to create your own right now, but it's one of the common renderers that should be shipped by default.

This renderer will need two modes (at least in case of keyboard):

1. Single key: single key
2. With modifier: single key with optional modifier

Both modes should show a human readable output in the final widget. For example, hitting alt then x should read ALT + x when in mode 2.