Units can be selected via 'u', 'j', 'v' and 'k'; full-screen
unit details view not supported.
Items can be selected via 't', 'k', 'v'->inventory.
Also, when viewing a container item full-screen, the selected
contained item or unit is returned; never the container itself.
The api is used in rename to allow setting nicknames for
arbitrary units, including animals and enemies.
Allow defining commands with guard conditions, and binding
one or more commands to alphabetic and function keys. When
the relevant key is pressed, the first listed command with
successfully evaluated guard is chosen.
For consistency, the guard is also checked when the command
is invoked from the console; this requires suspending the
core inside PluginManager, before invoking plugin code.