Add keyboard shortcut presets

[gregid]

Many languages use Alt as a letter modifier making the default shortcuts not working for these languages. The idea is to provide few presets that the user could choose and additionally ability to import keyboard presets from file.

Since zen is keyboard heavy it would make it much easier to recommend to users when they don't need to change every keyboard shortcut manually to make the browser work.

Additionally it would allow for better experimentation with the shortcuts as you could easily switch between presets