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.
As a usage example, allow toggling water level display and idlers,
and implement a ui tweak for easily copying stockpiles.
Also disable df2mc by default - default options shouldn't
require anything not in the base package.