C++ JoystickButton and POVButton were both nonfunctional due to slicing when trigger passes itself by value to the button scheduler it creates.
Fix is to remove the virtual Get() method entirely and use only the m_isActive functor; since the subclass now passes the button condition back as a functor to the base class, in which it's stored as a member, it will now still work after being sliced.
Assorted improvements to the ergonomics of declaring requirements in the new
command framework. C++ requirements list parameters have been defaulted
to an empty list, some missing C++ requirements list parameters have been
added, and both C++ and Java have been given requirements list params in
various InstantCommand wrapper methods (#2049), whose value is
forwarded to the command.
