The structure used to store commands has changed.
- Plain lists were abstracted away by using a DEFSTRUCT with
  appropriate options (esp. the :TYPE LIST option).
- Slots were given names matching the previously defined accessor
  functions.  This makes the rest of the code compatible with the
  previous version.
Image files will now be ignored by Git.
Merge remote-tracking branch 'origin/master':

Some time ago I've made some changes to this branch on another machine and
didn't merge them.  The time has come for those commits to land on my laptop.