Terminal layout

Support at least one layout that emulates a keyboard enough to use a terminal, i.e. have modifiers like ctrl, and arrow keys.

Open design questions:

• do we need separate terminal layouts for different languages?
• what keys do we actually need? Function keys? Alt? Compose? PrintScreen? Insert?
• do we need a different layout for not strictly terminal purposes, with a different set of keys (e.g. text editor keyboard)?