Rework custom shortcut prefs

Rework custom shortcut prefs

- Defaults are now set for all prefs (including custom shortcuts and advanced
  options), not just the options that can be seen in the options UI.
- All command prefs are now named 'mode.<mode>.<command>'. So are the
  translation IDs for the commands. This helps to simplify the code.
- Migration from the 0.5.x format and the format used in the devlopment version
  up until this commit can now be done once, instead of at every startup and
  every shortcut change.
- All old custom shortcut prefs are migrated automatically, using a generalized
  migration facility that can be reused for other migrations in the future.
- Some command names were slightly renamed to improve consistency and to make
  the more descriptive. The latter is important since regular users are now
  expected to be able to read lib/defaults.coffee as documentation on all
  default shortcuts.